Got a check engine light? Here’s a reference guide for the most common OBD-II trouble codes on full-size Chevy and GMC trucks — what they mean, what causes them, and what to do about them.
Most Common Codes on GM Trucks
P0300 — Random/Multiple Cylinder Misfire
What it means: The engine is misfiring — either randomly across multiple cylinders or at a rate that can’t be attributed to one cylinder.
Common causes:
- Spark plugs worn or fouled (most common)
- Ignition coil failure (one coil bad often causes multiple codes)
- Injector issue
- Low compression (more serious)
Fix: Replace spark plugs first — many P0300 issues resolve with fresh plugs. If it returns, check coils and pull individual cylinder misfire codes (P0301–P0308).
P0014 / P0013 — Exhaust Camshaft Position Actuator (Bank 1/2)
What it means: The variable valve timing (VVT) system is not responding correctly.
Common causes:
- Low oil level or dirty oil (most common)
- Stuck VVT solenoid (cam phaser actuator)
- Oil pressure issue
Fix: Change the oil first — low or old oil causes VVT sluggishness. If the code returns, replace the camshaft actuator solenoid (a $20–$40 part, ~30-minute job).
P0449 — EVAP System Vent Control Circuit
What it means: The evaporative emission vent solenoid has a circuit fault.
Common cause: Failed EVAP vent solenoid (located on or near the charcoal canister, usually rear frame area)
Fix: Replace the vent solenoid. Often a $15–$25 part.
P0442 / P0440 — EVAP Small/Large Leak
What it means: The evaporative emission system has detected a fuel vapor leak.
Common cause (P0442): Loose or damaged fuel cap — try tightening or replacing the gas cap first before anything else
Common cause (P0440): Larger leak; EVAP solenoids, purge valve, or cracked EVAP lines
P0128 — Coolant Below Thermostat Regulating Temperature
What it means: The engine is not reaching normal operating temperature.
Common cause: Thermostat stuck open (most common)
Fix: Replace the thermostat. Inexpensive and easy access on most GM V8s.
P0101 — Mass Airflow (MAF) Sensor Range/Performance
What it means: The MAF sensor reading is outside expected range.
Common cause: Dirty MAF sensor (contamination from oily filter on aftermarket intakes)
Fix: Clean the MAF sensor with MAF cleaner spray. If code returns, replace the sensor.
P0700 — Transmission Control System Malfunction
What it means: The TCM has detected a fault — this is a generic code that means pull the trans-specific codes.
Fix: Pull all codes — the P0700 is just a flag that more specific codes are stored.
Recommended Scanner
An OBD-II scanner like the Innova 5310 OBD2 Scanner will read and clear codes, show live data, and help you diagnose issues without a trip to the shop. Worth having in every truck.
Key Rule: Fix the Cause, Not the Code
Don’t just clear the code — diagnose what set it. Clearing a code without fixing the cause just delays the inevitable and may mask a developing problem.