Top 5 Kenworth T680 Problems I See as a Mobile Diesel Mechanic

The Kenworth T680 is one of the best-looking, most aerodynamic Class 8 trucks on the road. Drivers love the cab, fleet managers love the fuel economy, and Kenworth built a truck that competes head-to-head with anything Freightliner or Volvo puts out. But as a mobile diesel mechanic who works on T680s every week, I can tell you this truck has its weak spots — and most of them come down to the PACCAR MX-13 engine and the systems built around it. I’m Albert, owner of Albert’s Road Service in West Palm Beach, and the T680 is one of my most frequent service calls.

Whether you’re running a 2014 T680 with the original MX-13 or a 2024 with the latest updates, certain problems show up over and over. Here are the top 5 Kenworth T680 problems I fix as a mobile diesel mechanic, what causes them, what they cost, and how to prevent them.

1. PACCAR MX-13 Aftertreatment/DPF Derate Issues

This is the number one reason T680 owners call me. The PACCAR MX-13 aftertreatment system — the DPF, SCR catalyst, DEF dosing, and all the sensors that monitor emissions compliance — is the single biggest source of breakdowns, derates, and lost revenue on the T680. If you’ve driven a T680 for any length of time, you’ve seen that amber warning light and the derate countdown. It’s not a matter of if, it’s when.

What Happens

Your dashboard lights up with a check engine light followed by an aftertreatment derate warning. PACCAR’s system gives you a countdown — sometimes a few hundred miles, sometimes just a few hours — before the truck goes into a 5 mph derate or a forced idle shutdown. The MX-13’s emissions system is aggressive about enforcement, and unlike some other engines, PACCAR doesn’t give you a lot of grace period once the countdown starts. Drivers go from running full speed to being stuck on the shoulder of I-95 in a matter of hours.

Common Causes

• DPF soot loading and failed regens. The diesel particulate filter traps soot from the exhaust. Under normal operation, the engine performs active regenerations — burning off soot at high exhaust temperatures — to keep the DPF clean. But if the truck spends too much time idling, running short trips around Palm Beach County, or operating in stop-and-go traffic, the regens can’t complete. Soot builds up past the threshold, and the system forces a derate. I see this constantly on local and regional T680s that never get sustained highway runs. The fault codes you’ll see are SPN 3251 (DPF soot loading) and related ash loading faults.

• DEF quality sensor failures. The sensor in the DEF tank measures urea concentration. When it reads out of spec — either from a failed sensor or from contaminated DEF — the system assumes you’re running bad fluid and triggers a derate. In Florida’s heat, DEF degrades faster in the tank, especially if the truck sits for a few days. Bad DEF from contaminated storage tanks at truck stops is another major cause. SPN 4094 (DEF quality) is one of the most common fault codes I clear on T680s.

• SCR efficiency codes. The SCR catalyst converts NOx using DEF. When the system detects that NOx conversion isn’t meeting its target — measured by the outlet NOx sensor — it sets an SCR efficiency fault. This can be caused by a degraded catalyst, a failed DEF dosing injector, a clogged DEF line, or simply a NOx sensor that’s reading wrong. SCR efficiency derates are some of the hardest to diagnose because multiple components can cause the same code.

• DEF dosing unit clogging. The injector that sprays DEF into the exhaust stream gets clogged with crystallized urea. Florida’s heat accelerates this — ambient temperatures above 90 degrees cause DEF to crystallize faster in the lines and at the injector tip. When the dosing unit can’t deliver the right amount of DEF, the SCR can’t do its job, and you get a derate.

• Exhaust temperature sensor failures. The MX-13 has multiple exhaust temperature sensors throughout the aftertreatment system. When one fails or reads out of range, the system can’t manage regens properly. Failed regen attempts lead to soot buildup, which leads to derates. These sensors are relatively cheap but fail regularly from heat exposure.

What It Costs

• Forced DPF regen (on-site): $200-$400
• DPF cleaning (off-truck): $400-$800
• DEF quality sensor replacement: $300-$600 parts + labor
• DEF dosing unit cleaning/replacement: $500-$1,200
• NOx sensor replacement: $500-$900
• SCR catalyst replacement: $2,500-$5,000
• Full aftertreatment diagnostic: $150-$300

Prevention

• Use only name-brand DEF from sealed containers. Never buy DEF from an open drum or a truck stop with a questionable supply chain. Contaminated DEF is responsible for the majority of DEF-related derates I see.
• Don’t let the truck idle excessively. The DPF needs sustained highway-speed exhaust temperatures to regenerate. If your route is mostly local, make sure you get the truck on the highway periodically for a full regen cycle.
• Never cancel or interrupt an active regen. If the truck is parked regenerating, let it finish. Interrupted regens are the fastest path to a soot-loaded DPF.
• Stay current on PACCAR software updates. PACCAR releases aftertreatment calibration updates that improve regen strategy and sensor tolerances.
• If you get a warning, call me immediately at 561-475-8052. The sooner I can diagnose the issue, the more options you have before the derate gets worse.

2. EGR Cooler and EGR Valve Failures

The exhaust gas recirculation system on the PACCAR MX-13 is a known trouble area, especially on 2013-2019 model year engines. The EGR cooler recirculates a portion of exhaust gas back into the intake to reduce combustion temperatures and lower NOx emissions. It’s a critical emissions component, but it operates in one of the harshest environments on the engine — hot exhaust gas on one side, engine coolant on the other — and it fails more often than it should.

What Happens

The first sign is usually white smoke from the exhaust stack, especially at startup or under load. That white smoke is coolant being burned in the combustion process after leaking through a cracked EGR cooler. You might also notice the coolant level dropping without any visible external leak — the coolant is going into the exhaust and out the stack as steam. If the EGR valve is the problem instead, you’ll see rough idle, poor throttle response, and excessive black smoke from carbon buildup restricting airflow.

In severe cases, a cracked EGR cooler can leak enough coolant into the intake to hydro-lock a cylinder. I’ve seen MX-13 engines with bent connecting rods from coolant ingestion caused by a catastrophically failed EGR cooler. That’s an engine rebuild or replacement — a $25,000+ repair that started as a $3,000 cooler job.

Common Causes

• Thermal fatigue. The EGR cooler cycles between extreme temperatures thousands of times. The internal tubes expand and contract until they develop micro-cracks. This is a design limitation — the cooler simply wears out over time. PACCAR redesigned the cooler several times, but earlier versions (2013-2017) are particularly failure-prone.

• Carbon buildup on the EGR valve. The EGR valve opens and closes to regulate exhaust flow. Over time, soot and carbon deposits build up on the valve and in the EGR passages, causing the valve to stick open or closed. A stuck-open valve allows too much exhaust into the intake, causing rough running and power loss. A stuck-closed valve can cause the engine to run hotter than intended.

• Coolant contamination. If the coolant isn’t maintained properly — wrong concentration, depleted additives, or mixed coolant types — the coolant side of the EGR cooler corrodes faster. In Florida, I see this often because the heat breaks down coolant additives faster than in cooler climates.

• High idle time. Trucks that idle excessively run the EGR cooler at lower temperatures, which increases condensation inside the cooler. That condensation mixes with soot to form acidic deposits that eat through the cooler tubes from the inside.

What It Costs

• EGR cooler replacement: $2,500-$4,500 parts + labor
• EGR valve cleaning: $300-$600
• EGR valve replacement: $800-$1,500
• Coolant system pressure test and diagnosis: $150-$300
• Complete EGR system overhaul (cooler + valve + pipes): $4,000-$6,500

Prevention

• Test your coolant every 6 months. Keep the additive package at the correct level and make sure the concentration is right for your climate.
• Don’t mix coolant types. If you’re topping off, use the same brand and formula that’s already in the system.
• Minimize excessive idling. Use an auxiliary power unit (APU) or bunk heater instead of idling the engine for cab comfort.
• Have the EGR valve cleaned during major PM services — every 200,000 miles or so. A $500 cleaning can prevent a $4,000 cooler replacement.
• If you see white smoke or unexplained coolant loss, stop driving and call for engine diagnostics immediately. Catching a cracked EGR cooler early prevents catastrophic engine damage.

3. Coolant Crossover Tube Leaks

This is a well-known PACCAR MX-13 problem that Kenworth T680 owners learn about the hard way. The coolant crossover tube runs between the two cylinder heads on the MX-13, transferring coolant across the top of the engine. The original design used a tube that develops leaks at the O-ring seals or cracks at the connection points. It’s such a common failure that PACCAR redesigned the tube, but thousands of T680s are still running with the original part, and even the updated version isn’t immune.

What Happens

You’ll notice coolant dripping from the top-center of the engine, usually running down the back of the block. At first it’s a small weep — just enough to leave a stain or a faint smell of coolant when the engine is hot. Most drivers notice it during a pre-trip inspection when they see coolant residue on the engine. If ignored, the leak gets progressively worse. The O-rings harden and shrink from heat cycling, the sealing surfaces corrode, and what started as a drip turns into a steady stream.

The danger isn’t just the leak itself — it’s where the coolant goes. Coolant running down the back of the engine can reach the starter, the wiring harness, and electrical connectors. I’ve seen corroded starters and fried wiring harnesses caused by a crossover tube leak that went unrepaired for months. And if the truck loses enough coolant, you’re looking at overheating and potential engine damage.

Common Causes

• O-ring deterioration. The crossover tube uses O-ring seals at each connection point. These O-rings are exposed to constant heat cycling — hot when running, cold when parked — and they harden, crack, and lose their seal over time. In Florida’s heat, this process is accelerated. I typically see crossover tube leaks start around 300,000-500,000 miles.

• Corrosion at the sealing surfaces. If the coolant isn’t properly maintained, corrosion builds up on the sealing surfaces where the tube meets the heads. Even a new O-ring won’t seal properly against a corroded surface.

• Original tube design. The first-generation crossover tube used thinner material and a less robust seal design. PACCAR released an updated tube with better seals and material, but if your T680 is a 2013-2018 model and the tube has never been replaced, you’re probably running the original.

• Vibration and thermal expansion. The engine vibrates, the metal expands and contracts, and over hundreds of thousands of miles, the connections loosen. This is a mechanical wear issue that affects every MX-13 eventually.

What It Costs

• Crossover tube replacement (updated part): $800-$1,500 parts + labor
• O-ring replacement only (if caught early): $300-$600
• Coolant system flush and refill: $200-$400
• Secondary damage repair (corroded starter, wiring): $500-$2,000+

Prevention

• Inspect the crossover tube area during every pre-trip. Look for coolant residue, staining, or wetness on the top-center and back of the engine.
• Check coolant level regularly. An unexplained slow coolant loss often points to the crossover tube.
• If your T680 has the original crossover tube and is over 300,000 miles, consider proactive replacement with the updated PACCAR part during your next major service. It’s a lot cheaper to replace it on your schedule than to break down on the road.
• Maintain your coolant properly — correct concentration, fresh additives, and the right coolant type. Good coolant protects the sealing surfaces from corrosion.

4. Electrical/Wiring Harness Issues

The Kenworth T680 is a technology-forward truck with a complex electrical architecture. Multiple electronic control units, a fully digital dash, integrated telematics, and hundreds of sensors all communicate through an extensive wiring harness network. When it works, it’s impressive. When it doesn’t, the symptoms range from annoying to truck-down, and they can be maddening to diagnose.

What Happens

The most common complaint I hear from T680 owners is dashboard warning lights that come and go with no apparent pattern. The check engine light illuminates, you pull over, the light goes away. ABS lights that flash intermittently. Gauge readings that jump erratically. Lights that flicker or stop working, then start again. These “phantom faults” drive drivers crazy and can cost thousands in unnecessary parts replacements when the real problem is a $50 connector or a chafed wire.

In more severe cases, you get hard failures — the truck won’t start, the transmission won’t shift, the engine derate won’t clear, or the aftertreatment system throws codes that don’t make sense. I’ve had T680s towed in for what appeared to be major engine problems that turned out to be a corroded ground wire on the frame rail.

Common Causes

• Chassis harness chafing. The main wiring harness runs along the frame rails and through routing clips that are supposed to keep it secure. Over time, vibration loosens the clips, and the harness rubs against the frame, crossmembers, or other components. The wire insulation wears through, exposing bare wire that shorts to ground intermittently. This is the single most common electrical issue I find on T680s, and it’s almost always in a spot you can’t see without crawling under the truck.

• Connector corrosion. This is a Florida specialty. The salt air in Palm Beach County works its way into every electrical connector on the truck. The pins develop green corrosion that increases resistance, causing intermittent signals, voltage drops, and phantom fault codes. Under-cab and under-hood connectors are the worst, but I’ve found corroded connectors everywhere — even inside the cab on trucks parked near the coast.

• BCM (Body Controller Module) issues. The T680’s body controller module manages lights, gauges, HVAC, and accessory functions. When the BCM has internal faults or receives corrupted signals from corroded inputs, the symptoms are bizarre — lights that turn on by themselves, gauges that peg to max, horn that blows randomly. BCM problems usually require reprogramming or replacement, and the diagnostic time to confirm a BCM failure versus a wiring issue can be significant.

• Battery and charging system degradation. The T680’s electronics are sensitive to voltage. When batteries are weak or the alternator output drops, the low voltage causes ECMs and modules to behave erratically. In Florida, batteries last 2-3 years at best — the heat kills them. I replace more batteries on T680s in South Florida than almost any other component.

• Aftermarket accessory wiring. Aftermarket CB radios, GPS units, dash cameras, inverters, and lighting often get spliced into the factory harness by whoever installed them. Bad splices, improper wire gauges, and missing fuses introduce voltage spikes and ground faults into the factory electrical system. I’ve traced more phantom faults to aftermarket wiring than I can count.

What It Costs

• Battery replacement (pair): $400-$700
• Alternator replacement: $600-$1,200
• Wiring harness repair (chafed/damaged section): $300-$800
• Connector cleaning, repair, and sealing: $100-$400
• BCM replacement and programming: $1,500-$3,000
• Electrical diagnostic (mobile): $150-$300/hr
• Full electrical system inspection: $300-$600

Prevention

• Replace batteries proactively every 2-3 years in Florida. Don’t wait for a no-start situation.
• Apply dielectric grease to all accessible electrical connectors during preventive maintenance services. This seals out moisture and prevents corrosion.
• Inspect the chassis wiring harness for chafing during PM services. Pay attention to areas where the harness passes through the frame or near moving components.
• If you install aftermarket accessories, have them wired properly with dedicated circuits and fuses. Don’t splice into the factory harness.
• Address intermittent electrical issues immediately. A phantom fault today becomes a hard failure tomorrow — usually at 2 AM on the turnpike.

5. Air Suspension and Cab Ride Quality Problems

The T680 was designed for driver comfort, and Kenworth put a lot of engineering into the cab suspension and air ride system. But the air suspension components — bags, leveling valves, shock absorbers, and cab mounts — are wear items that degrade over time. In Florida, the heat accelerates rubber deterioration, and the constant sun exposure bakes air bags and bushings from the outside. I get steady calls from T680 owners dealing with ride quality problems, air leaks, and cab vibrations.

What Happens

The most noticeable symptom is a truck that won’t ride level. You’ll see one side sitting lower than the other, or the whole truck sitting low when parked. The air system can’t maintain height because air bags are leaking, leveling valves are stuck, or air lines have developed cracks. While driving, you’ll feel excessive bouncing, a harsh ride over bumps, or a swaying sensation that makes the cab feel unstable.

The T680 is also known for cab vibration issues at certain speeds — typically in the 55-65 mph range. Drivers describe it as a buzz or harmonic vibration through the cab that makes long hauls fatiguing. This is different from a tire balance or driveline vibration — it’s specifically a cab suspension issue related to worn shock absorbers and cab mounts.

Common Causes

• Air bag deterioration. The rubber air bags in the chassis suspension and cab suspension degrade from heat, UV exposure, and ozone. In Florida, I see air bags crack and leak at roughly half the mileage they’d last in a northern climate. The bags develop small cracks that grow until they leak air faster than the system can replenish it. You’ll hear the air compressor cycling constantly as it tries to keep up with the leaks.

• Leveling valve failures. The height control valves — also called leveling valves — regulate air pressure to maintain ride height. They have mechanical linkages that wear out, and the valve itself can stick from dirt, corrosion, or internal diaphragm failure. A stuck leveling valve causes the truck to ride too high, too low, or at different heights on each side.

• Cab air suspension mount wear. The T680’s cab is mounted on air springs and shock absorbers that isolate the cab from the chassis. The rubber bushings in these mounts harden and crack in Florida’s heat, and the mounting hardware loosens from vibration. Worn cab mounts are the primary cause of the harmonic vibration issue at highway speeds.

• Shock absorber degradation. The cab shocks and chassis shocks wear out faster in Florida because the heat breaks down the hydraulic fluid and degrades the seals. A blown shock absorber doesn’t just affect ride quality — it allows excessive movement that accelerates wear on air bags, bushings, and other suspension components.

• Air line and fitting leaks. The nylon air lines that connect the air bags to the leveling valves and supply tanks become brittle from heat exposure. They crack at connection points, especially where they pass through clips or tight bends. Air fittings develop slow leaks from vibration and corrosion. These small leaks are insidious — the truck seems fine when you start it, but after a few hours, it’s sitting low because the slow leak has drained the air bags.

What It Costs

• Chassis air bag replacement (per bag): $200-$500
• Cab air spring replacement (per spring): $300-$600
• Leveling valve replacement: $200-$400
• Cab shock absorber replacement (pair): $300-$600
• Cab mount bushing replacement (set): $400-$800
• Air line and fitting repair: $100-$300
• Complete cab suspension overhaul: $1,500-$3,000
• Full chassis air suspension service: $1,000-$2,500

Prevention

• Inspect air bags visually during every pre-trip. Look for cracks, bulges, and signs of rubbing. Catching a cracked bag early prevents a blowout on the highway.
• Listen for air leaks when the truck is parked with the engine off. A hissing sound means air is escaping somewhere — find it before it gets worse.
• Replace cab shock absorbers every 300,000-400,000 miles. Don’t wait for the ride to get terrible. Fresh shocks protect the rest of the suspension system and make a dramatic difference in driver comfort.
• Apply UV protectant to exposed rubber suspension components during PM services. It won’t make them last forever, but it slows the degradation in Florida sun.
• If you have the highway-speed cab vibration, don’t just live with it. Have the cab mounts and shocks inspected. The vibration causes fatigue for the driver and accelerates wear on the cab structure.

Bonus: The T680’s Hidden Weak Spots

A few more issues that don’t make the top 5 but are worth knowing:

• A/C compressor failures. Just like every truck in Florida, the T680’s A/C runs nearly year-round. Compressors, condensers, and evaporators wear out faster in South Florida’s climate. Budget for annual A/C service and don’t ignore weak cooling — a compressor that’s struggling wastes fuel and will eventually seize.
• Fan clutch issues. The MX-13’s viscous fan clutch can fail in either direction — locked on (wasting fuel and power) or locked off (engine overheats in traffic). Florida heat puts the fan clutch under constant demand, and they fail here more often than in cooler climates.
• Turbocharger actuator wear. The MX-13’s variable geometry turbo actuator is exposed to extreme heat and soot. While not as failure-prone as some competitor engines, the actuator linkage needs periodic cleaning and lubrication to prevent sticking.
• Air dryer cartridge neglect. Florida’s humidity means the air dryer works overtime. A saturated AD-SP cartridge lets moisture into the entire air system — corroding valves, air chambers, and fittings. Change the cartridge every 100,000 miles or annually, whichever comes first.
• Fifth wheel wear. The T680’s Holland or Fontaine fifth wheel needs regular greasing and inspection. In Florida’s sandy, salty environment, the pivot points and locking mechanism wear faster than usual. A worn fifth wheel is a safety issue and a DOT violation.

When to Call a Mobile Mechanic vs. Going to a Shop

Most of the top 5 problems listed here can be diagnosed and often repaired on-site by a mobile mechanic. Here’s my general rule:

Call me for mobile repair:

• Aftertreatment derates, DPF regens, and DEF system issues
• Battery, alternator, and starter replacements
• Coolant crossover tube replacement
• Electrical diagnostics and wiring harness repair
• Air suspension repairs — bags, shocks, leveling valves
• Brake repairs and ABS sensor replacement
• Aftertreatment/DPF repair on-site

Go to a shop for:

• EGR cooler replacement (major job, needs proper access)
• Internal engine work — injectors, head gasket, in-frame overhaul
• Major transmission repair or clutch replacement
• BCM replacement requiring factory programming tools
• Frame or structural welding

For everything else, call Albert’s Road Service at 561-475-8052. I’ll be straight with you about whether it’s a mobile repair or a shop job. I don’t waste your time or money.

Keep Your T680 Running in Florida

The Kenworth T680 is a fantastic truck — great cab, solid fuel economy, and a loyal following among owner-operators and fleets. But the PACCAR MX-13 engine has its quirks, and Florida’s climate puts extra stress on every system. The aftertreatment, EGR, coolant, electrical, and suspension components all need more attention down here than they would up north.

Stay ahead of maintenance, address problems at the first sign of trouble, and keep a good mobile mechanic’s number saved in your phone. A Kenworth repair specialist who knows the MX-13 inside and out can save you thousands in downtime and unnecessary shop bills.

Albert’s Road Service — 561-475-8052 — 24/7 mobile truck and trailer repair, West Palm Beach, FL.

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Albert is the owner of Albert’s Road Service LLC — a 24/7 mobile truck and trailer repair service based in West Palm Beach, Florida. He specializes in Freightliner, Kenworth, Peterbilt, Volvo, and International truck repair throughout Palm Beach County and South Florida. Call 561-475-8052.