You read your car’s codes and there it is: error code P0420. Don’t panic, it’s one of the most common codes. In this article, you’ll learn exactly what P0420 means, what the causes are, whether you can still drive, and how to fix it without immediately replacing the most expensive part.
What does error code P0420 mean?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." In plain language: the onboard computer sees that your catalytic converter on bank 1 is not working as well as it should. The car measures this with two oxygen sensors, one before and one after the catalytic converter. If those two signals are too similar, the computer concludes that the cat is not doing its job.
Important: P0420 points to an area, not an exact cause. The cat itself is often not the culprit.
Symptoms
Engine warning light is on. Often the only noticeable symptom. The car usually continues to drive normally.
Sometimes higher consumption or reduced performance. Not always present, but possible with a truly worn cat.
Risk during the MOT. A lit engine warning light and increased emissions can lead to failure.
Possible causes (from cheap to expensive)
Oxygen sensor after the cat. A slow or worn sensor often causes false P0420 codes. Relatively cheap to replace, check this first.
Exhaust leak near the sensors. A leak before or after the cat disrupts the measurement. Often inexpensive to fix.
Poor fuel or oil consumption. Contamination can temporarily cause the cat to measure poorly.
Worn catalytic converter. The most expensive cause. Only replace after cheaper options have been ruled out.
How to determine the cause
Use a scanner to read the live data from both oxygen sensors. The sensor before the cat should fluctuate quickly; the sensor after should be fairly stable. If both fluctuate about the same, the cat (or the sensor after it) is suspicious. Also visually check for exhaust leaks. This way you avoid replacing an expensive catalytic converter when a sensor costing just a few tens was the real culprit.
Which scanner do you use for this?
For reading and live data of the oxygen sensors, an affordable scanner is sufficient:
View the Topdon CarPal → 
View the OBDeleven 3 → 
View the Launch CRP919X →
The Topdon CarPal (around €40) reads the code and live data via your phone. The OBDeleven 3 (around €77) does that plus all modules. The Launch CRP919X (around €440) adds active tests.
Frequently Asked Questions
Can I keep driving with P0420?
Usually yes; the car often still drives fine. It’s not an immediate danger, but your emissions are increased and you might fail the MOT. Have it checked within a reasonable time.
Can I just clear P0420?
You can clear the code, but if the cause remains, the code will return after a few drives. So only clear it after you have addressed the cause.
Is it always the catalytic converter?
No. Very often it’s the oxygen sensor after the cat or an exhaust leak. Check those cheaper causes first before replacing an expensive catalytic converter.
What does fixing it cost?
An oxygen sensor or leak is relatively inexpensive. A new catalytic converter is much more expensive. That’s why it’s worth diagnosing properly first.
In short
P0420 means that your catalytic converter on bank 1 is underperforming according to the onboard computer. The cause is often an oxygen sensor or exhaust leak, not necessarily the catalytic converter itself. Read the live data before replacing anything. Check out the scanners in our diagnostic equipment collection or read how to read and clear fault codes.