P0420: Catalyst System Efficiency Below Threshold (Bank 1)

P0420 means the catalytic converter on Bank 1 (the engine side with cylinder 1) is no longer cleaning the exhaust efficiently enough, based on the computer comparing the oxygen sensors before and after it. The most common cause is a worn, aged catalytic converter. Usually there are no driveability symptoms beyond the warning light and slightly worse economy, but the car will fail emissions or TUV. A converter replacement typically costs 400 to 1800 EUR or more.

P0420 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0420 and illuminates the check engine light.

If your vehicle has triggered P0420, you may notice one or more of these symptoms:

• Check engine light illuminated, often the only obvious sign
• Slight drop in fuel economy
• Will fail an emissions or TUV inspection
• Occasional sulfur or rotten-egg smell from the exhaust
• Sluggish acceleration in rare cases where the converter is partially blocked
• No noticeable change in how the car drives in most cases

Here are the most common causes of P0420, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Generally safe for short-term driving. Extended driving may damage the catalytic converter further and will fail emissions testing.

Follow these steps to pinpoint the root cause of P0420:

1. Read freeze frame data to see the engine load, temperature and fuel trims when the code was stored
2. Scan for other codes first, misfire (P0300 series), O2 sensor or fuel-trim codes usually point to the real root cause and should be fixed before touching the converter
3. Inspect the exhaust from the manifold back, look and listen for leaks at gaskets, flex pipes and welds before and around the converter
4. Watch live O2 sensor data, the upstream sensor should swing rapidly while a healthy downstream sensor stays nearly flat
5. Compare upstream versus downstream waveforms, if the downstream sensor swings almost as much as the upstream one, the converter is failing
6. Check fuel trims and the MAF reading, rule out a rich or lean condition or a dirty MAF that could be contaminating the converter
7. Clear the code and complete a drive cycle, confirm whether it returns and re-test catalyst monitor readiness

Repair costs for P0420 vary depending on the root cause and your vehicle. Here are typical estimates:

P0420 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0420 or indicate related issues: