P0421
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
- Severity
- Medium
- Driving
- Drivable — book it in
The warm-up catalyst on bank 1 is not converting as well as it should. This one sits close to the engine so it reaches working temperature quickly after a cold start, which is when a petrol engine produces most of its emissions - so it does a disproportionate share of the work despite being the smaller unit.
Possible causes
- CommonFailing or aged downstream oxygen sensor
- CommonExhaust leak before or between the oxygen sensors
- CommonEngine fault upstream that has damaged the converter - misfire, rich mixture, oil burning
- CommonConverter genuinely worn out
- PossibleOxygen sensor contaminated by oil, coolant or silicone sealant
- PossibleAftermarket converter that does not meet the original specification
What you might notice
- Check engine light on
- Failed emissions test
- Slight loss of power
- Sulphurous or rotten-egg smell from the exhaust
- Rattling from the converter if its internals have broken up
- Often no symptom the driver notices
What to check, in order
- Do not replace the converter first. It is the most expensive item in the exhaust and the oxygen sensors around it fail far more often.
- Check for exhaust leaks before and between the sensors. Air drawn in through a leak makes a working converter measure badly.
- Watch the upstream and downstream oxygen sensors together on the live data screen. A downstream sensor that mirrors the upstream one suggests the converter really has stopped working; one that sits still suggests the sensor has.
- Read all codes. Misfire or fuel trim codes alongside this one identify what damaged the converter, and replacing it without fixing them repeats the expense.
- Ask whether an aftermarket converter has been fitted. Cheap replacements frequently fail this test within months even when new.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Auxiliary emission controls
P0400P0401P0402P0403P0404P0405P0406P0407P0408P0409P0410P0411P0412P0413P0414P0415P0416P0417P0418P0419P0420P0421P0422P0423P0424P0425P0426P0427P0428P0429P0430P0431P0432P0433P0434P0435P0436P0437P0438P0439P0440P0441P0442P0443P0444P0445P0446P0447P0448P0449P0450P0451P0452P0453P0454P0455P0456P0457P0458P0459P0460P0461P0462P0463P0464P0465P0466P0467P0468P0469P0470P0471P0472P0473P0474P0475P0476P0477P0478P0479P0480P0481P0482P0483P0484P0485P0486P0487P0488P0489P0490P0491P0492P0493P0494P0495P0496P0497P0498P0499