What does code P0305 mean on a GMC and is it safe to drive?
P0305: Cylinder 5 Misfire Detected. Here's what it means on your GMC, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
What this code means
Misfire on cylinder 5—present on V6, V8, and inline-5 engines only. On V6 engines, cylinder 5 is sometimes harder to access; factor labor into repair estimates.
Common causes for your GMC
- Fouled spark plug
- Failed ignition coil
- Leaking or clogged injector
- Low compression from valve or ring wear
Symptoms
- Check Engine Light
- Engine shaking or rough idle
- Power loss under acceleration
Typical fixes
- Replace spark plug and coil on cylinder 5
- Test and replace injector if needed
- Compression and leak-down test
Repair cost range
For a GMC, repairs for P0305 often fall between $150 and $1000, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on GMC
GMC Sierra 5.3L P0305 is an AFM deactivation cylinder—lifter collapse is the primary suspect on high-mileage trucks. On the 4.2L inline-six Envoy, P0305 cannot occur (only 6 cylinders active). On the 5.3L, an AFM delete kit prevents recurrence.
Most commonly affected models