2021 GMC P0303: What It Means & Is It Safe to Drive?
P0303 on the 2021 GMC: Cylinder 3 Misfire Detected. Here's what it means, what usually causes it, what repairs typically cost, and whether it's safe to keep driving.
What this code means
The PCM has detected a misfire specifically on cylinder 3. Diagnosis follows the same steps as P0301—swap parts to the same position on a known-good cylinder to isolate the fault.
Common causes on the 2021 GMC
- Worn or fouled spark plug on cylinder 3
- Failing ignition coil on cylinder 3
- Leaking or clogged fuel injector
- Low compression on cylinder 3
Symptoms
- Check Engine Light, often flashing under load
- Rough idle or shaking
- Loss of power, hesitation
Typical fixes
- Replace spark plug and coil on cylinder 3
- Test and replace fuel injector
- Compression test if ignition parts don't resolve it
Repair cost range
For a 2021 GMC, repairs for P0303 typically fall between $150 and $900, depending on the root cause and labor rates in your area.
Known issues on GMC
GMC P0303 on the 5.3L V8 in Sierra and Yukon is an active cylinder—standard coil and plug diagnosis applies. On the 3.6L LLT V6 in Acadia, P0303 on the rear bank typically comes from carbon buildup on the intake valves given the direct injection design.
Most commonly affected models