Here are the most common codes and what they mean.
1. P0171 – System Too Lean
Meaning: Too much air, not enough fuel
Causes:
- Vacuum leak
- Dirty MAF sensor
- Weak fuel pump
Fix: Clean MAF, check vacuum hoses
2. P0300 – Engine Misfire
Meaning: Random misfire
Causes:
- Spark plugs
- Ignition coil
- Injector problem
Fix: Replace plugs and coils
3. P0420 – Catalytic Converter Efficiency Low
Meaning: Catalytic converter failure
Fix: Check O2 sensor, exhaust leak, or replace catalytic
4. P0A80 – Replace Hybrid Battery (Toyota)
Meaning: HV battery weak
Fix: Replace modules or battery pack
5. C1391 – Brake Booster Pump Leak (Toyota Prius)
Meaning: Internal leak in brake accumulator
Fix: Replace brake booster pump assembly
6. P0500 – Vehicle Speed Sensor
Meaning: Speed signal lost
Causes:
- ABS sensor
- Wiring
Fix: Replace wheel speed sensor