P0421Powertrain
Powertrain · SAE
Warm-Up Catalyst Efficiency Below Threshold — Bank 1
Meaning
The catalyst monitor detects that the bank 1 warm-up (light-off) catalytic converter does not reduce emissions sufficiently during the warm-up phase. Comparison of upstream and downstream sensor signals indicates reduced conversion efficiency.
Common causes
- Aged or poisoned warm-up catalyst (sulfur, lead, silicone contamination)
- Thermal damage to the monolith substrate from prolonged rich mixture
- Faulty or incorrectly positioned downstream O2 sensor
- Rich-mixture excursion during cold start that damaged the catalyst
- Incorrectly calibrated upstream AFR sensor that skews the test
Symptoms
- Malfunction indicator lamp (MIL) on
- Elevated HC and CO emissions especially during cold phase
- Engine performance generally normal
- Possible hydrogen sulfide odor (rotten eggs) from exhaust
- Sometimes no noticeable driveability symptoms
Severity
Moderate: engine performance remains acceptable, but emissions exceed regulatory limits and the catalyst may deteriorate further.
What to do
- Run the catalyst monitor with a scan tool and check OSC (oxygen storage capacity) values
- Check the condition and operation of both upstream and downstream sensors
- Inspect the warm-up catalyst for physical damage or blockage
- Rule out misfire or rich mixture that may have damaged the catalyst
- Replace the warm-up catalyst if efficiency loss is confirmed