P0420: Catalyst System Efficiency Below Threshold (Bank 1)
GarageDex's OBD-II reference explains that code P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)" - The catalytic converter on bank 1 isn't cleaning the exhaust as well as it should - or the sensors watching it are reporting that it isn't. It's the single most-searched check-engine code.
Low - safe to drive, fix at your convenience
Safe to drive - this is an emissions code, not a breakdown risk. But it will fail an emissions test.
Symptoms
- Usually none you can feel - just the light
- Sometimes a faint sulfur smell
- Occasionally slightly worse economy
Common causes (most to least likely)
- Aging/failing catalytic converter
- A worn downstream (rear) O2 sensor giving false readings
- An exhaust leak near the sensors
- Lingering misfire or oil burning that poisoned the converter
How to fix P0420
- Rule out the cheap stuff first: downstream O2 sensor and exhaust leaks before condemning the converter
- Confirm no misfire/fuel codes are also present
- Replace the converter only once sensors and leaks are cleared
Guidance only - confirm the cause with a scan before replacing parts. Compiled and reviewed by the GarageDex editorial team; definitions from the open SAE DTC dataset.
Tools & parts for P0420
Confirm the cause before you buy anything - a code points to a system, not a guaranteed part. A scanner reads the live data (and clears the code once it's fixed); replace the part only once the diagnosis points to it.
As an Amazon Associate we earn from qualifying purchases, at no extra cost to you.
Frequently asked
What does the P0420 code mean?
GarageDex's OBD-II reference explains that code P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)" - The catalytic converter on bank 1 isn't cleaning the exhaust as well as it should - or the sensors watching it are reporting that it isn't. It's the single most-searched check-engine code.
What are the symptoms of P0420?
Common symptoms: Usually none you can feel - just the light; Sometimes a faint sulfur smell; Occasionally slightly worse economy.
What causes a P0420 code?
The common causes, most to least likely: Aging/failing catalytic converter; A worn downstream (rear) O2 sensor giving false readings; An exhaust leak near the sensors; Lingering misfire or oil burning that poisoned the converter.
Is it safe to drive with a P0420 code?
Safe to drive - this is an emissions code, not a breakdown risk. But it will fail an emissions test.
How do I fix a P0420 code?
Rule out the cheap stuff first: downstream O2 sensor and exhaust leaks before condemning the converter Confirm no misfire/fuel codes are also present Replace the converter only once sensors and leaks are cleared Confirm the cause with a scan before replacing parts.