OBD-II trouble code
P0422: Main Catalyst Efficiency Below Threshold (Bank 1)
The PCM has decided the main catalytic converter on bank 1 isn't cleaning the exhaust well enough. It's a close relative of the far more common P0420 — same monitor, same two-sensor logic — but the wording points specifically at the main catalyst brick. Rule out a lazy downstream oxygen sensor and an exhaust leak before the converter is replaced. Those checks are in the procedure because they produce the same pattern, and they are the smaller repairs.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $2,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0422 mean?
P0422 comes from the same catalyst-monitoring strategy that produces P0420. Where the upstream sensor is a switching zirconia sensor, the PCM compares that swing with the downstream sensor behind the converter. A wideband upstream sensor does not switch. A healthy catalyst stores and releases oxygen, which smooths the downstream signal so it barely moves. As the converter's coating degrades, it loses that storage capacity and the downstream sensor begins to mirror the upstream one. When the rear sensor tracks the front too closely, the PCM concludes catalyst efficiency has dropped below the threshold its calibration allows and sets the code. The 'main catalyst' wording in P0422 simply specifies which catalyst element in the bank-1 exhaust the monitor is judging.
A misfire, a rich condition, or oil or coolant in the exhaust can damage the catalyst that this monitor is judging. Walker Exhaust says periodic shell temperatures above 1600°F can damage the coating. Repair those stored faults before a converter is installed, or the replacement sees the same exhaust. A slow downstream oxygen sensor and an exhaust leak ahead of that sensor produce a pattern the monitor can store as low efficiency. Check them before the converter is condemned.
The code is an efficiency result. It blocks an OBD inspection while it is stored. Restricted flow is a broken substrate, which is a separate complaint from a low efficiency number. The sequence is misfires and fuel trims, the downstream sensor, exhaust leaks, then the converter.
Common causes
- Slow or biased downstream (post-cat) oxygen sensor faking a low-efficiency reading
- Exhaust leak ahead of the rear O2 sensor letting outside air skew the signal
- Upstream misfire baking raw fuel into the catalyst
- Engine running rich from a fuel or sensor fault, overheating the converter
- Main catalyst with low oxygen-storage capacity, judged only after the downstream sensor and the exhaust leak have been checked
- Oil or coolant consumption coating and poisoning the substrate
- Aftermarket or low-quality converter that can't meet the efficiency threshold
Symptoms
- Check engine light with P0422 stored
- No drivability change from the efficiency result itself
- Failed emissions or smog inspection
- Possible faint sulfur or rotten-egg smell from the exhaust
- Sluggishness or reduced power only if the converter is physically breaking down
- Sometimes accompanied by P0420 or related fuel-trim codes
Diagnostic steps
- 1.Scan for any misfire, lean, or rich codes first and fix those — they cause catalyst failure and must be resolved before judging the converter.
- 2.Inspect the exhaust for leaks between the engine and the rear O2 sensor; an air leak there mimics low catalyst efficiency.
- 3.Verify the downstream O2 sensor itself before using it to judge the converter. A lazy or biased rear sensor produces the same signal pattern as a spent catalyst, so a conclusion drawn from an unverified sensor is worth nothing — and the sensor is a fraction of the price of the part it would condemn.
- 4.Compare upstream and downstream data only where the upstream sensor is a switching zirconia sensor. Bosch describes that cell from about 0.1 volts lean to about 0.9 volts rich. A wideband air-fuel sensor does not switch; use the oxygen-storage test in the service information instead. Bosch's ME7 catalyst monitor measures oxygen storage against a borderline catalyst, and that threshold is the calibration. A cold or idling comparison is not that test.
- 5.Check fuel trims for a chronic rich condition that would damage a new converter.
- 6.Only after the above, consider catalyst replacement — and use a converter that meets OEM efficiency specs.
Repair cost
$150 – $2,500
The low end of the printed bracket is the downstream oxygen sensor or the exhaust leak, after that part has failed its test. The high end is the main catalyst. Read the bracket as a planning span, not a quoted price. A converter installed behind a misfire or a rich condition that is still present sees the same exhaust that damaged the last one.
Estimate your repair
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Open the Repair Cost Estimator with catalytic converter replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.