The steady check engine light on your 2022 Buick Regal accompanied by code P0420 indicates your catalytic converter isn't cleaning exhaust gases as efficiently as expected. Before panicking about expensive converter replacement, it's worth understanding what can trigger this code—because sometimes the converter itself isn't the real problem.
What P0420 Indicates
The PCM compares readings from the upstream (before converter) and downstream (after converter) oxygen sensors. A healthy converter causes the downstream sensor to show a relatively steady signal while the upstream fluctuates. When the downstream sensor begins mimicking the upstream's fluctuations, the PCM determines converter efficiency has dropped below threshold.
Common Causes
Actual Converter Degradation
Catalytic converters do wear out over time. The catalyst material degrades from heat cycles, contamination, or age. This is what the code is designed to detect.
Oxygen Sensor Problems
A lazy or failing downstream O2 sensor can provide data that resembles converter inefficiency even with a healthy converter. Sensors are much cheaper than converters and should be evaluated first.
Exhaust Leaks
Leaks between the engine and downstream sensor allow atmospheric oxygen into the exhaust, confusing sensor readings and triggering efficiency codes.
Engine Running Issues
Long-term rich or lean conditions stress the converter. Running rich can overheat and damage the catalyst; lean conditions cause elevated temperatures. Fix the underlying cause before replacing the converter.
Oil or Coolant Contamination
If engine problems allow oil or coolant into the exhaust stream, these substances coat the catalyst and reduce efficiency.
Diagnosis
- Check for other codes: Misfire codes, fuel system codes, or O2 sensor codes indicate underlying problems.
- Inspect O2 sensors: Compare upstream and downstream sensor response and test response time.
- Check for exhaust leaks: Listen and look for leaks between the engine and downstream sensor.
- Monitor fuel trims: Long-term fuel trims beyond ±10% indicate mixture problems damaging the converter.
- Temperature test: A working converter should be hotter at the outlet than inlet. Significantly cooler outlet suggests diminished efficiency.
Repair Options
Address Underlying Issues First
Fix any misfires, fuel system problems, or exhaust leaks before condemning the converter.
O2 Sensor Replacement
If the downstream sensor is sluggish, replacement may resolve the code. Cost: $150-$300.
Catalytic Converter Replacement
If the converter is genuinely failed, replacement is necessary. Cost: $1,000-$2,500 for OEM-quality replacement.
Emissions Warranty
Federal emissions warranty covers catalytic converters for 8 years/80,000 miles. Your 2022 Regal should be covered if within these limits and failure wasn't caused by neglect or modifications.