P0302 on GMC: How to Diagnose & Fix
Cylinder 2 Misfire Detected
2 GMC models · 2 documented issues · 1 critical · $1,200-$3,500 repair
P0302 on GMC by model: symptoms, diagnosis and fix
The LML Duramax EGR (exhaust gas recirculation) cooler is prone to coolant leaks internally, allowing coolant to enter the intake manifold and combustion chambers. Coolant in the intake causes white smoke, rough running, and potential hydrolocking of cylinders. The EGR valve also sticks open or closed from carbon buildup, causing rough idle, black smoke, and check engine lights. The EGR cooler failure can occur as early as 60,000 miles on trucks used in stop-and-go driving. GM bulletin 08-06-04-003D provides the EN-48974 adapter set for pressure-testing the EGR cooler for coolant leaks into the combustion chambers.
Common Symptoms
- White smoke from exhaust especially on startup
- Coolant loss with no visible external leak
- Rough idle or engine misfire
- Check engine light with EGR codes (P0401, P0404)
- Coolant smell from exhaust
- Overheating or coolant in oil (severe cases)
- Black smoke and poor fuel economy (EGR valve stuck open)
How to Fix
Replace EGR cooler with updated GM design or aftermarket unit (e.g. BulletProof Diesel LML EGR cooler). Clean or replace EGR valve. Flush cooling system and inspect for coolant contamination in intake. Consider EGR delete kit for off-road/race use only (illegal for street use). Prevent premature failure by maintaining proper coolant pH and avoiding extended idle periods.
Sources for this issue (2)
GM says major carbon build-up on the intake and/or exhaust valves, from fuel contamination or incomplete burning of the fuel, can cause rough idle, extended crank, crank-no-start or misfire with codes P0300-P0308, and that valves may stick. 2014-2018 GMC Sierra 1500 gasoline engines (4.3L V6, 5.3L V8, 6.2L V8); GM bulletin 16-NA-383 (June 2024) covers all 2000-2025 GM gasoline passenger cars and light/medium-duty trucks.
How to Fix
Have a technician follow the service-manual misfire diagnosis. If carbon is the cause and there is no valve damage, bulletin 16-NA-383 calls for a Top Engine Cleaner procedure through the throttle body to free sticking valves and remove carbon. GM also advises Top Tier gasoline and regular oil changes to reduce build-up. Normal warranty rules apply; this is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
P0302 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0302 mean on a GMC?
P0302 stands for "Cylinder 2 Misfire Detected." This code means the engine computer has detected a misfire specifically in cylinder 2, meaning that cylinder isn't completing combustion properly on some firing events. The computer watches tiny variations in crankshaft speed to spot when a cylinder fails to fire, and a misfire means the air-fuel mixture isn't igniting correctly — usually from a spark, fuel, or compression problem in that cylinder. You may feel a stumble, rough idle, shaking, loss of power, or a flashing check-engine light, and persistent misfires can damage the catalytic converter. Because it names a single cylinder, the fault is often a component serving just cylinder 2, like its coil, plug, or injector. On GMC it is documented across 2 models: Sierra 2500HD, Sierra 1500.
What are the most common causes of P0302?
The most common causes of P0302 are: Worn or fouled spark plug (cylinder 2), Failed ignition coil or coil-on-plug (cylinder 2), Clogged or faulty fuel injector (cylinder 2), Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/bent valve, head gasket), Damaged spark plug wire or boot (if equipped), Carbon-tracked or cracked coil/plug causing spark loss, Faulty PCM/ECM driver (rare). The specific cause varies by vehicle.
How much does it cost to fix P0302 on a GMC?
Published repair estimates on GMC range from $1,200 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.