P0423 — Main Catalyst Efficiency Below Threshold (Bank 1)
Drive with caution — fix soon.
What P0423 means
This code means the main catalytic converter on bank 1 has dropped below its required efficiency, based on the oxygen sensors before and after it. While a worn-out catalyst is a common cause, it can also stem from a faulty oxygen sensor, an exhaust leak, or an engine condition like a misfire. Repairs range from sensor replacement to replacing the main catalytic converter.
Common symptoms
- Check engine light is on
- Failed emissions or smog test
- Reduced fuel economy
- Sulfur or rotten-egg odor from exhaust
- Slight loss of engine power
Common causes
- Worn or failing main catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak ahead of or near the catalyst
- Engine misfire or rich fuel condition
- Catalyst contaminated by burning oil or coolant
Typical repair cost
$250 – $1800
Varies by vehicle and root cause.
Frequently asked questions
Can I keep driving with a P0423 code?
Usually yes, at least short-term. The car typically runs normally, but emissions are higher and it will fail a smog test. Get it diagnosed soon so a minor cause doesn't turn into a major converter replacement.
How much does it cost to fix a P0423 code?
It ranges widely, from about $250 for a sensor or exhaust-leak fix to $1,800 or more if the main catalytic converter needs replacement, which is the priciest scenario.
Is a P0423 code serious?
It's moderate severity. It won't strand you right away, but it points to an emissions problem, and if the cause is a misfire or rich mixture it can keep damaging the catalyst.
What is the 'main' catalyst in P0423?
It refers to the primary catalytic converter on bank 1, the larger converter that does most of the emissions cleanup, as opposed to a smaller warm-up catalyst nearer the engine.