What is the difference between P0420 and P0422?

Both indicate Bank 1 catalyst efficiency problems. P0420 is the generic OBD-II code used by most manufacturers. P0422 is used by some manufacturers (commonly Chrysler-Jeep and some European brands) to specifically label the main catalyst when the exhaust has a separate warm-up pre-cat and main cat. The diagnosis is essentially identical.

Will P0422 cause my car to fail an emissions inspection?

A stored catalyst efficiency code with the check engine light on is exactly the kind of defect that gets a car rejected at a periodic technical inspection. Each country sets and grades its own test, so there is no single European verdict, but do not count on it going unnoticed: fix the fault and let the readiness monitors complete before you book.

Should I try an O2 sensor before replacing the catalyst?

Yes, because the downstream O2 sensor is roughly five to ten times cheaper than the converter. If sensor switching is slow but other readings are normal, replacing the sensor first is the sensible step.

How long should a replacement main catalyst last?

It depends far more on the engine than on the converter itself. Fitted to a healthy engine, a main catalyst can last for many years and high mileage. Fitted to an engine that is still running rich or burning oil, the replacement gets poisoned the same way the original did and can fail again quickly, so sort out any fuel-trim or oil-consumption problem before spending money on the cat.

P0422: Main Catalyst Efficiency Below Threshold (Bank 1)

Quick Answer

Safe to drive (short-term)

P0422 means the main catalytic converter on Bank 1 has dropped below the efficiency threshold the ECU expects. It is similar to P0420 but used on some manufacturers (notably Chrysler, Jeep, and certain European platforms) to distinguish the main cat from the upstream warm-up cat in multi-brick exhaust systems.

Severity: Moderate · Emissions
Estimated repair cost: From €105 to €1660+, varies by vehicle

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What does P0422 mean?

P0422 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0422 and illuminates the check engine light.

Where it is on the car

Engine
Oxygen sensor before the converter (sensor 1)
Catalytic converter
Oxygen sensor after the converter (sensor 2, or 3 on some cars)
Silencer and tailpipe

What are the symptoms of P0422?

If your vehicle has triggered P0422, you may notice one or more of these symptoms:

Check engine light illuminated
Slight drop in fuel economy
Rotten-egg sulfur smell from exhaust
Possible rattling from under the car (substrate breaking up)
Failed emissions or periodic technical inspection

What causes P0422?

Here are the most common causes of P0422:

Failing or worn main catalytic converter (Bank 1)
: high
Faulty downstream O2 sensor (Bank 1, Sensor 2) with sluggish switching
: medium
Exhaust leak between pre-cat and main cat or after the main cat
: medium
Prior misfire damaging the main converter brick
: low
Oil or coolant consumption fouling the substrate
: low

Is it safe to drive with P0422?

Generally yes, for short-term driving. Safe for normal driving. A lit engine warning light is checked at the periodic roadworthiness inspection in most EU countries and can be recorded as a deficiency, which may result in a failed inspection; each country applies its own rules. If the code is accompanied by rattling under the floor, the converter substrate may be breaking apart internally and replacement should be prioritised.

How do you diagnose P0422?

Follow these steps to pinpoint the root cause of P0422:

Read freeze frame data and check for companion catalyst codes (P0420, P0421) that may be active at the same time
Inspect the full Bank 1 exhaust run for leaks, especially at flange joints and slip-fit connectors after the main cat
Graph the Bank 1 downstream O2 sensor at steady 2,000 to 2,500 RPM. A healthy main cat keeps it stable around 0.4 to 0.6 V
If the downstream signal swings more than 0.2 V repeatedly, the main cat is no longer storing oxygen and is failing
Test downstream sensor switching speed. Anything over 300 ms is sluggish and may be the actual fault

How much does P0422 cost to fix?

Repair costs for P0422 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair	Parts Cost	Labor Cost	Total Estimate	DIY Difficulty
Replace main catalytic converter (Bank 1)	€220–€1400	€90–€260	€310–€1660	Professional
Replace downstream O2 sensor (Bank 1)	€55–€160	€60–€130	€115–€290	Moderate
Fix exhaust leak (gasket or pipe section)	€25–€120	€80–€220	€105–€340	Moderate

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0422 on specific vehicles

P0422 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

P0422 on BMW

BMW 3 Series

See BMW-specific causes & cost

P0422 on Volkswagen

Volkswagen Golf

See Volkswagen-specific causes & cost

P0422 on Audi

Audi A6

See Audi-specific causes & cost

P0422 on Mercedes-Benz

Mercedes-Benz E-Class

See Mercedes-Benz-specific causes & cost

These codes are often seen alongside P0422 or indicate related issues:

Common questions about P0422

What is the difference between P0420 and P0422?: Both indicate Bank 1 catalyst efficiency problems. P0420 is the generic OBD-II code used by most manufacturers. P0422 is used by some manufacturers (commonly Chrysler-Jeep and some European brands) to specifically label the main catalyst when the exhaust has a separate warm-up pre-cat and main cat. The diagnosis is essentially identical.
Will P0422 cause my car to fail an emissions inspection?: A stored catalyst efficiency code with the check engine light on is exactly the kind of defect that gets a car rejected at a periodic technical inspection. Each country sets and grades its own test, so there is no single European verdict, but do not count on it going unnoticed: fix the fault and let the readiness monitors complete before you book.
Should I try an O2 sensor before replacing the catalyst?: Yes, because the downstream O2 sensor is roughly five to ten times cheaper than the converter. If sensor switching is slow but other readings are normal, replacing the sensor first is the sensible step.
How long should a replacement main catalyst last?: It depends far more on the engine than on the converter itself. Fitted to a healthy engine, a main catalyst can last for many years and high mileage. Fitted to an engine that is still running rich or burning oil, the replacement gets poisoned the same way the original did and can fail again quickly, so sort out any fuel-trim or oil-consumption problem before spending money on the cat.

Read our detailed guides

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