P0420 flags catalyst efficiency, P0171 flags a lean fuel mixture and P0455 flags a large evaporative leak. Each code names a system, not a part. Ruling out the cheap causes first saves money.
Key takeaways
- Codes name a system.
- Rule out cheap causes first.
- Confirm with testing.
P0420
Catalyst system efficiency below threshold. Check for exhaust leaks, oxygen sensors and misfires before replacing the converter.
P0171
System too lean. Look for vacuum leaks, a dirty mass airflow sensor and fuel delivery.
P0455
Large evap leak. Start with the gas cap, then hoses and valves.
How codes are structured
The first letter names the system, and P is powertrain. The digits narrow the category.
Next step
Bring the codes and symptoms.
Why P0300 series codes are different
Codes from P0300 to P0308 are misfire codes. P0300 means random misfires, while P0301 means cylinder one, P0302 cylinder two and so on. Knowing the cylinder narrows the cause. If the same cylinder repeats, the plug, coil or injector for that cylinder is the likely culprit. If several cylinders misfire, look at fuel, air and compression.
What to do with pending codes
A pending code means the computer saw a fault once but has not confirmed it. The light may not be on yet. Pending codes are an early warning. A technician can read them even when no light is lit. If a smog check is coming, a quick scan for pending codes can catch a problem before it becomes a failure.
Need help in south Bakersfield?
Questions about check engine light? Read our Check Engine Light page, call (661) 599-9098 or request a time. Peña's Auto Electric, Smog & Repair, 303 Taft Hwy, Suite B, Bakersfield, CA 93307. Mon to Fri 9:30 AM to 5:30 PM, Sat 8 AM to 1 PM.
Frequently asked questions
What does P0420 mean?
Catalyst efficiency below threshold.
What causes P0171?
Vacuum leaks, sensors, fuel.
What causes P0455?
A large evap leak.
Can I clear codes?
Not before a visit.