P0266 on GMC: How to Diagnose & Fix
Cylinder 2 Contribution/Balance Fault
1 GMC model · 1 documented issue · 1 critical · $600-$5,000 repair
P0266 on GMC by model: symptoms, diagnosis and fix
The fuel injectors on the Duramax LML (Bosch piezoelectric) and L5P (Denso solenoid) engines can fail due to contaminated fuel, high-mileage wear, or internal electrical faults. A failed injector causes white smoke on startup, rough idle, misfires on the affected cylinder, and fuel dilution of the engine oil. The high fuel rail pressures (about 29,000 PSI on the L5P) stress the injectors over time. Contaminated fuel from bulk diesel stations accelerates injector wear.
Common Symptoms
- White or gray smoke on cold start
- Rough idle or misfire on one or more cylinders
- Fuel smell in engine oil (fuel dilution)
- Check engine light with injector circuit codes
- Reduced power and poor fuel economy
- Hard starting in cold weather
How to Fix
Diagnose the failed injector with a relative compression test and injector balance rate scan using a Tech 2 or equivalent scan tool. Replace the failed injector(s) — OEM injectors are $300-$500 each, and labor is 2-4 hours per injector ($300-$600). Always replace the injector hold-down clamp and fuel return line seals. If one injector fails at high mileage, consider replacing all 8 as the others are likely near end of life. Use a quality fuel additive like Stanadyne or Hot Shot Secret to protect remaining injectors.
Sources for this issue (2)
P0266 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 P0266 mean on a GMC?
P0266 stands for "Cylinder 2 Contribution/Balance Fault." P0266 is a generic powertrain code indicating the engine computer detected that cylinder 2 is not contributing its expected share of power, meaning its output is unbalanced compared to the other cylinders. The PCM monitors crankshaft acceleration and fuel/injector behavior per cylinder, and an imbalance here often signals a misfire, fuel-delivery, or mechanical problem specific to cylinder 2. Left unaddressed, this can cause rough running, poor performance, and possible catalytic converter damage. On GMC it is documented across 1 model: Sierra 3500HD.
What are the most common causes of P0266?
The most common causes of P0266 are: Faulty or clogged fuel injector on cylinder 2, Injector wiring or connector fault, Ignition fault (coil, plug) causing a cylinder-2 misfire, Low compression / mechanical issue (valve, ring) on cylinder 2, Vacuum or intake leak affecting that cylinder, Contaminated or restricted fuel supply, PCM driver circuit fault for that injector. The specific cause varies by vehicle.
How much does it cost to fix P0266 on a GMC?
Published repair estimates on GMC range from $600 to $5,000 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.