P0422Powertrain
Powertrain · SAE
Main Catalyst Efficiency Below Threshold — Bank 1
Meaning
The catalyst monitor detects that the bank 1 main catalytic converter does not reach the minimum required efficiency. Oxygen storage capacity (OSC) is below the regulatory threshold.
Common causes
- Aged or depleted main catalyst due to high mileage
- Poisoning by contaminants (oil combustion, coolant ingestion, silicon-based additives)
- Thermal damage from misfire events or prolonged rich mixture
- Faulty downstream O2 sensor generating a false negative in the test
- Exhaust leak upstream of the downstream sensor altering the comparison
Symptoms
- Malfunction indicator lamp (MIL) on
- Elevated HC, CO, and NOx emissions
- Engine performance generally normal
- Possible pungent odor from exhaust
- Sometimes no noticeable driveability symptoms
Severity
Moderate to high: emissions exceed legal limits and a damaged catalyst can deteriorate further until it affects driveability.
What to do
- Check OSC values and upstream/downstream sensor signals with a scan tool
- Verify that both O2 sensors are functional and correctly positioned
- Perform back-pressure test to rule out physical blockage
- Rule out misfire, oil, or coolant ingestion as root cause of catalyst damage
- Replace the bank 1 main catalyst if insufficient efficiency is confirmed