Main Catalyst Efficiency Below Threshold (Bank 1)
Catalytic converter on bank 1 is not reducing emissions as expected.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The catalytic converter on bank 1 isn't cleaning exhaust gases properly anymore, which could lead to higher emissions and failed inspections.
What the computer is actually seeing
The engine's control unit checks the voltage levels from the front and rear oxygen sensors. If the difference in these readings indicates that the converter isn't working effectively, it sets this code.
What a healthy reading looks like
Front O2 sensor: 0.1V to 0.9V swing; Rear O2 sensor should have a smaller swing, typically between 0.1V to 0.5V with minimal fluctuation when the converter is operational.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Inspect the catalytic converter visually for any damage or physical obstruction. Decision: Is it damaged? If yes, replace the converter; if no, go to step 2.
- 22. Check for exhaust leaks before the catalytic converter. Decision: Any leaks found? If yes, repair leaks; if no, go to step 3.
- 33. Use a scan tool to monitor live data from the front and rear O2 sensors. Decision: Is the rear O2 sensor voltage significantly fluctuating? If yes, replace the rear O2 sensor; if no, go to step 4.
- 44. Check fuel trim data (short-term and long-term) for excessive positive or negative values. Decision: Is there an abnormal fuel trim? If yes, diagnose fuel system issues; if no, suspect a faulty catalytic converter.
Common causes
- Faulty catalytic converter
- Damaged or leaking exhaust system
- Faulty O2 sensors
- Wiring issues between the O2 sensors and ECU
- Engine running rich or lean conditions
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's moderate severity, but if power loss or rough running occurs, it's better not to drive until repaired.
Will this problem come back after repair?
If all related issues are fixed, it should not return. Ensure the engine runs properly to avoid damaging the new converter.
Why is this paired with P0420?
Both codes deal with catalytic converter efficiency, but P0422 specifically indicates a below-threshold performance on bank 1.
Working a real vehicle right now?
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