P0423
Heated Catalyst Efficiency Below Threshold Bank 1
Catalyst / emissions
Generic OBD-II
Medium
What it means
P0423 means the heated catalytic converter on bank 1 isn't cleaning the exhaust as well as it should — the ECM compares the upstream and downstream oxygen sensors and finds the converter below its efficiency threshold. It usually points to a worn-out converter, or an upstream problem (misfire, rich running, a bad O2 sensor) that damaged it.
Medium
Can I keep driving? You can keep driving — the car will fail emissions and may feel slightly sluggish, and a rattling converter needs attention, but it won't leave you stranded.
Common symptoms
What you'll notice
- Check-engine light on
- Failed emissions test
- Rattle from the converter (if it's breaking apart)
- Slightly reduced power if the converter is clogging
Common causes
Most likely first
- Worn-out or failed catalytic converterLikely
- Upstream misfire or rich mixture that overheated the converterCommon
- Faulty oxygen sensor(s)Common
- Exhaust leak near the converterLess often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and fix any misfire or fuel-trim codes first — they damage the converter.
- Check the upstream and downstream oxygen sensor readings.
- Inspect for exhaust leaks near the converter.
- Confirm the converter is genuinely the cause before replacing it (it's expensive).
- Replace the converter if worn, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0423?+
Yes for now, but don't ignore it — a converter breaking apart can rattle and eventually restrict the exhaust.
Can I just clear P0423?+
It returns within a few drive cycles unless the converter or the root cause is fixed.
Related codes
Often seen with P0423
Read & clear P0423 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX