P0422: Catalyst 2 Efficiency Below Threshold, Bank 1
P0422 means the second catalytic converter on bank 1 (the side of the engine with cylinder 1), usually the underfloor one, is no longer cleaning the exhaust well enough. It turns the check engine light on and can cause an emissions test failure, but the data lists no limp mode for it.
You can drive, but get it checked soon
The data lists no limp mode for this code, so the car normally keeps running. The converter is not cleaning the exhaust properly, though, and the cause should be found soon, especially since a prior misfire or mixture fault can be behind it and may keep causing damage.
What it means
A catalytic converter is a part in the exhaust that converts harmful gases into less harmful ones. Catalyst 2 is the second one in the exhaust flow on bank 1, the main or underfloor converter behind the one mounted close to the engine. Older documents call it the Main Catalyst. According to the reference, this code is documented only on GM's small diesels. On those engines the computer judges the underfloor oxidation catalyst separately from the close-coupled one (code P0421) by measuring the heat balance while the diesel particulate filter is cleaning itself (regeneration). An exhaust fuel injector sits ahead of the catalyst for this. Gasoline engines with several catalysts usually set P0420 instead.
Likely causes
- Main or underfloor catalyst aged or poisonedVery likely
The catalyst wears out or gets contaminated over time and then no longer converts enough, which is exactly what this code reports.
- Diesel: exhaust fuel injector coked, doses too littlePossible
If the injector ahead of the catalyst is clogged with carbon deposits, it delivers too little fuel, the catalyst does not heat up as expected, and the efficiency check fails.
- Temperature or oxygen sensor after the catalyst wrongPossible
The computer judges the catalyst from sensor readings, so a faulty sensor can make a working catalyst look bad.
- Misfire or mixture fault as prior damageLess likely
Engine misfires or a wrong fuel mixture can damage the catalyst or upset its readings.
What you may notice
- Check engine light on
- Emission test problems
- Diesel: possibly more frequent filter regenerations
What to check, cheapest first
- 1Read all stored codes and note any related ones such as P0421, P0420, P0432, P20CB or P0544, and any misfire or mixture codes. · you can do it
- 2Look at the exhaust system under the car for damage, leaks or loose connections near the catalyst and its sensors. · you can do it
- 3Fix any misfire or mixture faults first, then clear the code and see whether it returns. · mechanic
- 4Check the live readings of the temperature or oxygen sensor after the catalyst to see whether they are plausible. · mechanic
- 5On a diesel, inspect the exhaust fuel injector and test whether it delivers the right amount of fuel. · mechanic
- 6If the sensors and injector check out, test the catalyst itself and replace it if it no longer converts enough. Afterward, complete a drive cycle so the computer can rerun its test. · mechanic
Related codes
Frequently asked questions
Can I keep driving with P0422?+
Usually yes for a short time, since the data lists no limp mode. Have it checked soon, because a misfire or mixture fault behind it can cause more damage.
Can a loose gas cap cause P0422?+
The reference does not list the gas cap as a cause. It points to the catalyst, the exhaust fuel injector on diesels, a sensor after the catalyst, or an earlier misfire or mixture fault.
Will P0422 pass an emissions test?+
Probably not. The code is emissions relevant and turns the check engine light on, so expect a problem at the test until it is repaired.
Can I fix P0422 myself?+
The reference rates the repair as medium difficulty, about 1 to 3 hours, and says it is not suited to DIY. Reading codes and looking over the exhaust are reasonable owner steps.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10