P0420: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means the engine computer has decided the catalytic converter on bank 1 is no longer cleaning the exhaust well enough. The car usually drives normally, but the check engine light stays on and the car can fail an emissions test.
You can drive, but get it checked soon
The code itself usually causes no change in driving, so short-term driving is generally possible. If the catalyst has broken up (rattling, loss of power) or a misfire is the cause, continued driving can do more damage, and the car will fail an emissions test either way.
What it means
The catalytic converter (catalyst) is a part in the exhaust that converts harmful gases into less harmful ones. Bank 1 is the side of the engine that contains cylinder number 1. On gasoline engines the engine computer compares the oxygen sensor before the catalyst with the one after it. A healthy catalyst smooths out the signal of the rear sensor. If the rear signal moves too much like the front one, the computer sets P0420. On diesel engines the code refers to the oxidation catalyst, which the computer checks by how much heat it produces during filter cleaning (regeneration). The code turns on the check engine light and is relevant to emissions. After a repair, the car needs to complete a drive cycle (a set of normal driving conditions) before the computer can confirm the fix.
Likely causes
- Catalyst aged or poisoned (oil consumption, silicone, coolant)Very likely
Oil, silicone or coolant reaching the exhaust coats the catalyst and reduces its ability to convert gases; oil-borne poisoning is named as the most common cause.
- Catalyst overheated by misfire or rich runningPossible
Unburned fuel burning inside the catalyst can overheat and damage it.
- Rear oxygen sensor sluggish, dirty or unsuitablePossible
A slow or wrong sensor can give a signal that makes a working catalyst look bad.
- Exhaust leak ahead of or at the rear sensorPossible
Leaking air changes what the sensor reads and can distort the comparison.
- Diesel: oxidation catalyst sooted or poisoned, post-injection faultyPossible
If the catalyst is clogged or the extra fuel injection for regeneration fails, it produces too little heat.
What you may notice
- Check engine light on, otherwise often no change in driving
- Failed emissions test (higher HC, CO and NOx)
- Rattling or loss of power if the catalyst has broken up
What to check, cheapest first
- 1Read the code and any other stored codes. Misfire codes such as P0300 or rear oxygen sensor codes such as P0136 should be fixed first. · you can do it
- 2Listen and look for exhaust leaks near the manifold, the front pipe and the rear oxygen sensor (ticking, soot marks, hissing). · you can do it
- 3Check for oil or coolant consumption, since these can poison the catalyst and need to be fixed or the new catalyst will fail too. · you can do it
- 4Look at live data from the front and rear oxygen sensors to see whether the rear signal is flat or mirrors the front. · mechanic
- 5Test or replace the rear oxygen sensor if it responds slowly, is dirty or is the wrong type. · mechanic
- 6Replace the catalyst only after the other causes are ruled out, then complete a drive cycle to confirm the code does not return. On diesels, check the post-injection and the oxidation catalyst condition instead. · mechanic
Related codes
Frequently asked questions
Can I keep driving with P0420?+
Usually yes for a short time, since driving often feels normal. Have it checked soon, and stop if you hear rattling, lose power or notice misfires.
Can a loose gas cap cause P0420?+
The data does not list the gas cap as a cause. A loose cap is more typically linked to fuel vapor codes, so look at the catalyst, the rear oxygen sensor and exhaust leaks instead.
Will a car with P0420 pass an emissions test?+
Probably not. The code is emissions relevant, the check engine light is on, and a weak catalyst can show higher HC, CO and NOx readings.
Can I fix it myself?+
Some of it, yes. Repair difficulty is medium and DIY is possible, with an estimated 0.5 to 3 hours depending on whether it is a sensor, a leak or the catalyst.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10