P0431Powertrain
Powertrain · SAE
Warm-Up Catalyst Efficiency Below Threshold — Bank 2
Meaning
The catalyst monitor detects that the bank 2 warm-up (light-off) catalytic converter does not reduce emissions sufficiently during the warm-up phase. Bank 2 is the side opposite cylinder number 1.
Common causes
- Aged or poisoned bank 2 warm-up catalyst
- Thermal substrate damage from misfire events on bank 2
- Faulty or incorrectly positioned downstream O2 sensor bank 2
- Rich-mixture excursion during cold start on bank 2
- Upstream AFR sensor bank 2 drift that skews the test
Symptoms
- Malfunction indicator lamp (MIL) on
- Elevated emissions especially during cold phase
- Engine performance generally normal
- Possible hydrogen sulfide odor (rotten eggs) from bank 2 exhaust side
- Sometimes no noticeable symptoms
Severity
Moderate: bank 2 emissions exceed regulatory limits; risk of progressive deterioration if untreated.
What to do
- Run the catalyst monitor for bank 2 with a scan tool and check OSC values
- Check the condition and operation of bank 2 upstream and downstream sensors
- Inspect the bank 2 warm-up catalyst for physical damage or blockage
- Rule out misfire or rich mixture on bank 2 as root cause
- Replace the bank 2 warm-up catalyst if efficiency loss is confirmed