P0203 on GMC: How to Diagnose & Fix
Injector Circuit/Open - Cylinder 3
2 GMC models · 2 documented issues · 2 critical · $600-$6,500 repair
P0203 on GMC by model: symptoms, diagnosis and fix
The 2001-2004 6.6L Duramax RPO LB7 uses injectors buried under the valve covers that are notorious for failing well before 150,000 miles. GM's own analysis attributed the failures to fuel injector body cracks, ball seat erosion, and high-pressure seal extrusion. A leaking injector dumps unburned diesel into the cylinder and past the rings into the crankcase, raising the oil level and diluting it. Owner reporting on duramaxforum.com and silveradosierra.com consistently puts first failures in the 100k-150k range, and because the injectors sit under the valve covers the labor to reach them is far higher than on the later LLY/LBZ engines, where injectors are accessible from the top. This is an LB7-only failure mode - the LLY and later injectors were redesigned and do not share it.
Common Symptoms
- White or blue smoke at idle once warmed up
- Strong raw fuel smell from the exhaust
- Engine oil level rising between changes and smelling of diesel
- Rough idle and misfire, especially cold
- Hard starting
- Knocking/rattle under load
How to Fix
Replace all eight injectors as a set (replacing one at a time invites repeat teardown labor), along with the injector return lines and valve cover gaskets. GM issued a Special Policy extending injector coverage to 7 years/200,000 miles for qualifying trucks after a dealer balance-rate test - that program has long expired, so repairs are now owner-paid. Reman Bosch or new injectors plus the required ECM injector-code programming is the correct repair; change the oil immediately afterward because of the fuel dilution.
Sources for this issue (3)
The Bosch piezoelectric fuel injectors on the Duramax L5P and LML 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 (up to 36,000 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 Bosch 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 (1)
P0203 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 P0203 mean on a GMC?
P0203 stands for "Injector Circuit/Open - Cylinder 3." This code means the Engine Control Module detected an electrical problem (open circuit or out-of-range resistance) in the fuel injector control circuit for cylinder 3. The ECM pulses each injector electrically and watches the circuit to verify it is operating. P0203 means the expected feedback for cylinder 3's injector is missing or abnormal, so that cylinder may not be fueling properly. The typical result is a misfire, rough idle, and reduced performance traceable to cylinder 3. On GMC it is documented across 2 models: Sierra 2500HD, Sierra 3500HD.
What are the most common causes of P0203?
The most common causes of P0203 are: Damaged or corroded injector connector at cylinder 3, Open or shorted wiring in the injector control circuit, Failed (electrically open) fuel injector on cylinder 3, Loose or backed-out connector pin, Poor ground or power feed to the injector, Faulty injector driver inside the ECM/PCM, Chafed harness causing intermittent open. The specific cause varies by vehicle.
How much does it cost to fix P0203 on a GMC?
Published repair estimates on GMC range from $600 to $6,500 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.
Looking for P0203 on a different make?
View P0203 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.