OBD-II · P0420
P0420 Code: Catalytic Converter Efficiency Low — Do You Need a New One?
Catalyst system efficiency below threshold (Bank 1)
The short version
Yes, you can generally keep driving — it rarely causes drivability problems — but it will fail an emissions test and can point to underlying engine issues worth checking.
- Severity: Moderate
- Most common cause: Upstream misfire history that gradually damaged the converter
- 5 diagnosis steps in order
- 4 symptoms tied to this code
Darker bars are the more common causes; bar length is the top expected repair cost. Start with the dark, cheap causes before the expensive ones.
Can I keep driving?
Yes, you can generally keep driving — it rarely causes drivability problems — but it will fail an emissions test and can point to underlying engine issues worth checking.
Symptoms
- Check engine light on, usually with no obvious drivability symptoms
- Occasional sulfur/rotten-egg smell from exhaust
- Slight decrease in fuel economy
- Failing periodic vehicle emissions inspection
Causes & repair cost
Upstream misfire history that gradually damaged the converter
Most common$900 – $2,500
Failing upstream or downstream oxygen sensor
Most common$150 – $400
Genuinely aged/worn-out catalytic converter
Possible$900 – $2,500
Exhaust leak before or after the converter
Possible$100 – $400
Low-quality fuel or oil contamination fouling the catalyst
Rare$0 – $50
The converter itself is the expensive part; OEM units cost far more than aftermarket ones, and prices vary a lot by vehicle. Always fix the root cause first so the new converter doesn't fail again quickly.
Estimate the repair for your carDiagnosis steps in order
- Check for other stored codes (misfires, sensor faults) first
- Live-data scan upstream and downstream O2 sensor readings
- Inspect exhaust system for leaks
- Compare sensor waveforms — near-identical signals confirm converter inefficiency
- Fix any root cause (plugs, sensor) before replacing the converter
FAQ
Can I drive with a P0420 code?
Yes, usually there's no immediate danger, but the car will fail an emissions inspection.
Do I always need to replace the catalytic converter?
No, sometimes it's a failing O2 sensor or an exhaust leak, which is much cheaper to fix.
Why does the code come back after replacing the converter?
Usually because the root cause, like a misfire, wasn't fixed before the swap.
How much does a catalytic converter replacement cost?
It varies by vehicle and whether you use OEM or aftermarket parts, roughly $900-2500 at an independent shop.
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