What does P0420 mean?
The computer compares the oxygen sensors before and after the catalytic converter. A healthy converter stores oxygen, so the downstream sensor should stay fairly steady. When the downstream signal starts copying the upstream one, the computer decides the converter on bank 1 is not cleaning the exhaust well enough.
P0420 is one of the most common check engine codes in the US. It does not always mean you need a new converter. A good share of P0420 codes trace back to an exhaust leak, a tired downstream oxygen sensor, or an engine problem that is upsetting the readings. Rule those out before spending four figures.
Codes are a starting point. They tell you which test failed, not which part to replace. Check for other stored codes and fix those in a sensible order.
Symptoms
- Check engine light
- Usually no noticeable change in how the car drives
- Possible rotten egg smell
- Failed emissions test
- Loss of power only if the converter is physically clogged
Common causes
- Worn-out catalytic converter (common past 100,000 miles)
- Exhaust leak near the downstream O2 sensor
- Lazy or failing downstream oxygen sensor
- Misfires or oil burning that damaged the converter
- Engine running rich over time
How to diagnose P0420
- Check for misfire, fuel trim and O2 sensor codes first. Fix those before blaming the converter.
- Look at the downstream O2 sensor at a steady 2,500 rpm when fully warm. It should stay steady around 0.5 to 0.8 V. If it switches as fast as the upstream sensor, the converter is likely worn.
- Check for exhaust leaks around the sensors and flanges.
- Check for a software update (TSB) for your vehicle before buying a converter.
How to fix it
- Repair exhaust leaks
- Fix misfires or rich running first
- Replace a faulty downstream O2 sensor
- Replace the catalytic converter (use a CARB-compliant one in California and other CARB states)
Estimated repair cost
Likely repairs for P0420 run from about $100 to $3,500, depending on the cause.
| Repair | Typical range (parts + labor) |
|---|---|
| Diagnostic fee (scan + testing) | $100 - $200 |
| Oxygen / A/F sensor replacement | $150 - $450 |
| Exhaust leak repair (gasket/flange) | $100 - $400 |
| Catalytic converter (aftermarket, direct fit) | $900 - $2,500 |
| Catalytic converter (OE or CARB-legal) | $1,500 - $3,500 |
These are rough US estimates for an independent shop in 2026, based on typical parts prices and standard labor times. Dealers, big trucks, luxury brands and high-cost areas often run higher. They are not quotes. Use the repair cost estimator to adjust for your vehicle.
How serious is P0420?
Moderate severity
Usually safe to drive in the short term. Do not ignore a flashing check engine light or a red-hot converter, which means misfires are destroying it.
Make-specific notes
Toyota / Lexus
Many Toyotas use an air-fuel ratio sensor upstream and a regular O2 sensor downstream. Check both, and check for Toyota service bulletins and extended emissions warranty coverage on the converter.
Honda / Acura
Honda has released software updates for P0420 on some models. Ask a dealer to check for an ECM update before replacing the converter.
Ford
On engines with two banks, P0420 points at bank 1, the side with cylinder 1. Check the downstream sensor wiring, which can rub on the exhaust.
GM (Chevy, GMC)
On trucks, an exhaust leak at the manifold-to-pipe joint is a frequent cause. Fix leaks first, then re-run the catalyst monitor.
Always check open recalls and technical service bulletins for your VIN.
P0420 FAQ
Can I fix P0420 without replacing the catalytic converter?
Sometimes. If the cause is an exhaust leak, a bad downstream O2 sensor, a misfire or a software issue, fixing that clears the code. If the converter itself is worn out, the only real fix is a new converter.
Will a catalytic converter cleaner fix P0420?
Additives rarely fix a worn converter. They will not repair a converter that has lost its catalyst material or is physically damaged. They are cheap to try, but do not count on them.
Are O2 sensor spacers or defoulers legal?
Devices that trick the downstream sensor are considered emissions tampering under the federal Clean Air Act and fail many state inspections. We do not recommend them.
Is the catalytic converter covered by warranty?
Federal law requires the catalytic converter on 1995 and newer light-duty vehicles to be covered for 8 years or 80,000 miles. California and some other states have longer coverage on some vehicles. Check your warranty booklet.
What does P0420 mean?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)". The computer compares the oxygen sensors before and after the catalytic converter.
Can I drive with a P0420 code?
Usually safe to drive in the short term. Do not ignore a flashing check engine light or a red-hot converter, which means misfires are destroying it.
How much does it cost to fix P0420?
It depends on the cause. The likely repairs for P0420 range from about $100 to $3,500 at an independent shop in the US, plus a diagnostic fee of roughly $100-$200. These are rough estimates; dealer prices and some vehicles run higher.
What is the most common cause of P0420?
The most common causes are: worn-out catalytic converter (common past 100,000 miles); exhaust leak near the downstream O2 sensor; lazy or failing downstream oxygen sensor.