P2423
HC Adsorption Catalyst Efficiency Below Threshold Bank 1
Catalyst / emissions
Generic OBD-II
Medium
What it means
P2423 means the hydrocarbon (HC) adsorption catalyst on bank 1 — the part of the converter that traps unburned hydrocarbons during cold start — is no longer storing and converting them efficiently enough. The PCM detects this by comparing sensor readings. The engine runs normally, but the vehicle emits more hydrocarbons and will fail an emissions test until the cause is addressed.
Medium
Can I keep driving? Yes — the car drives normally. This is a catalyst-efficiency emissions fault; it won't strand you but will fail an emissions test and should be diagnosed.
Common symptoms
What you'll notice
- Check-engine light on — usually the only symptom
- Will likely fail an emissions or smog test
- Possible faint exhaust odor
- No noticeable change in power in most cases
Common causes
Most likely first
- Aged or failing HC adsorption catalyst/converter on bank 1Likely
- Upstream problem contaminating the catalyst (misfire, rich/lean running, oil or coolant burning)Common
- Faulty oxygen or temperature sensor feeding the efficiency calculationCommon
- Exhaust leak near the catalyst or its sensorsLess often
How to diagnose & fix
Work through it in order
- Check for and repair any upstream faults first (misfires, fuel-trim, O2 sensor codes) that damage the catalyst.
- Inspect the exhaust for leaks around the catalyst and sensors.
- Verify the relevant O2/temperature sensors are reading accurately.
- Replace the HC adsorption catalyst/converter if it has genuinely degraded.
- Clear the code and confirm the catalyst monitor passes.
FAQ
Quick answers
Is it safe to drive with P2423?+
Yes, it doesn't affect driveability, but it means higher emissions and a failed smog test. Diagnose the cause before replacing the converter.
Can I just clear P2423?+
It returns once the catalyst monitor runs unless the underlying efficiency problem is fixed.
Related codes
Often seen with P2423
Read & clear P2423 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