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. The honest reality is that a worn converter is the usual answer, but it's worth ruling out a lazy downstream O2 sensor and exhaust leaks first, because those are far cheaper.
Quick facts
- System
- Powertrain
- Category
- Emissions / Catalyst
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $200 – $2,500
- DIY difficulty
- Intermediate DIY
What does P0422 mean?
P0422 comes from the same catalyst-monitoring strategy that produces P0420. The PCM compares the upstream oxygen sensor (switching rapidly as it measures raw exhaust) against the downstream sensor behind the converter. 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 federal threshold and sets the code. The 'main catalyst' wording in P0422 simply specifies which catalyst element in the bank-1 exhaust the monitor is judging.
The trap with any catalyst-efficiency code is assuming the converter is dead and replacing it first. A converter rarely fails on its own — it's usually poisoned or overheated by an upstream problem. A chronic misfire dumps raw fuel that bakes the catalyst; an engine running rich from a bad sensor or leaking injector overwhelms it; oil or coolant entering the combustion chamber coats the substrate and kills it. So a converter that reads inefficient is often the *symptom* of something upstream that, left unfixed, will destroy the replacement too. Before condemning the cat, the monitor's own inputs deserve scrutiny: a slow or biased downstream O2 sensor can fake a low-efficiency reading, and an exhaust leak ahead of the rear sensor lets in outside air that skews it.
P0422 won't strip drivability — the car runs and drives normally — but it's an emissions failure that will block a smog/inspection pass and, on a genuinely failed converter, can eventually feel sluggish if the substrate breaks apart and restricts flow. The right sequence is: confirm no misfires or fuel-trim problems, verify the downstream O2 sensor and look for exhaust leaks, and only then weigh converter replacement.
Common causes
- Aged or contaminated main catalytic converter that has lost oxygen-storage capacity
- 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
- 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 noticeable change in how the car drives in most cases
- 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.Watch upstream and downstream O2 sensor live data: the rear sensor should stay relatively flat. If it swings in step with the front, the catalyst is likely degraded.
- 4.Verify the downstream O2 sensor responds correctly and isn't lazy or biased, which can false-set the code.
- 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
$200 – $2,500
If the cause is a downstream O2 sensor, expect roughly $200-$400. Repairing an exhaust leak is often $150-$400. A catalytic converter replacement is the expensive outcome — commonly $900-$2,500 depending on whether it's an OEM or aftermarket unit and how integrated it is with the exhaust. Always fix the underlying cause first; a new converter installed behind an untreated misfire or rich condition won't last.
Estimate your repair
Run the numbers for your vehicle
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.