P0422 Main catalytic converter not cleaning exhaust well enough

Main catalyst efficiency below threshold

Monitor

Kelley Blue Book says the car can be driven for some time with this code, though not for the long term, and the related catalyst efficiency codes on this site are also rated monitor.

It gets more serious if:

  • Inspect soon: Inspect soon if the engine loses power, or you smell rotten eggs or the exhaust glows or rattles, since a failing converter can clog.

Source: how we rate severity

P0422 in short

P0422 means the main catalytic converter is not cleaning the exhaust as well as the computer expects. The converter itself is the usual cause, but a failing oxygen sensor, an exhaust leak or engine problems such as oil burning can set it. You can drive for a while, but fix it before the converter clogs.

The computer compares the signals from the oxygen sensors before and after the catalytic converter. If the rear sensor swings like the front one, the converter is no longer storing oxygen and the efficiency code sets. California rules require this catalyst monitoring.

KBB lists the converter first, then oxygen sensors, the coolant temperature sensor, the fuel pressure regulator, oil contamination, exhaust leaks and wiring.

GM has bulletins that list P0422 for 2020 and 2024 Silverado and Sierra HD diesel trucks, and Audi lists it for 2000 A6 and A8.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Worn-out main catalytic converter

    Common general cause
    Why it ranks here
    KBB lists the converter first, and the CARB OBD-II rules require the computer to flag a converter that no longer cleans the exhaust.
    What would change this
    Rises if the rear oxygen sensor mirrors the front sensor with good sensors and no leaks. Falls if sensors or leaks are found.
    How to test the catalytic converter

  2. Oxygen sensor fault or contamination

    Common general cause
    Why it ranks here
    KBB lists the oxygen sensors among the causes since the monitor depends on their readings.
    What would change this
    Rises if a sensor is slow or stuck on a scan tool. Falls if both respond normally.
    How to test the oxygen (o2) sensor

  3. Exhaust leak near the sensors

    Common general cause
    Why it ranks here
    KBB lists exhaust leaks, which let outside air reach the sensor and look like a converter problem.
    What would change this
    Rises if you hear ticking or find soot at a joint. Falls if the exhaust is tight.

  4. Oil, coolant or rich-running contamination of the converter

    Possible — needs testing
    Why it ranks here
    KBB lists oil contamination, the fuel pressure regulator and the coolant temperature sensor as causes that damage the converter or confuse the monitor.
    What would change this
    Rises if oil burning, coolant loss or a rich mixture is found. Falls if the engine runs clean.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes and freeze-frame data. Note any misfire, fuel trim or oxygen sensor codes stored with P0422.

    Low riskTool: OBD-II scan tool

  2. Look and listen for exhaust leaks before the converter and at the sensors while the engine is cold.

    Use caution

    Warning:The exhaust is hot after driving. Work only on a cold car.

  3. Watch the front and rear oxygen sensor signals with live data at a steady speed.

    Low riskTool: scan tool with live data

Symptoms

  • Check engine light, car runs normally
  • Rotten egg smell from the exhaust Gets more serious: stop and have it checked.
  • Loss of power as the converter clogs Gets more serious: stop and have it checked.

Compare with: Check engine light with loss of power

How to fix

  1. With the engine off and cold, look and listen for exhaust leaks and fix any loose clamps.

Parts and cost context

  • Catalytic converter:Converters are expensive. Rule out leaks and oxygen sensors, and check any warranty coverage, before buying one. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Chevrolet

  • 2024 · Silverado 2500 HD

    Supported by vehicle data

    GM PIP5917 and 24-NA-058 cover P0422 on 2024 HD diesel trucks. The NHTSA summaries do not give the repair.

  • PIP5917 (chevrolet)Status unverified

    Direction when P0422 sets on a 6.6L L5P diesel

    2024silverado-1500, silverado-2500-hd, silverado-3500-hd · NHTSA 10236166

    Preliminary information for technicians; the NHTSA summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • 24-NA-058 (chevrolet)Status unverified

    MIL with P0422 and other DTCs

    2024silverado-2500-hd, silverado-3500-hd · NHTSA 10254028

    Repair information for a MIL concern with a list of DTCs including P0422.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Audi

  • 0401Status unverified

    MIL on with P0422 and/or P0432

    2000a6, a8 · NHTSA 10011565

    The NHTSA summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0420Catalytic converter efficiency below the computer's limit (Bank 1)

    Same system Common general cause

    P0420 is the generic catalyst efficiency code for bank 1; the causes and tests are the same.

  • P0421Warm-up catalytic converter not cleaning well enough

    Same system Common general cause

    P0421 is the warm-up catalyst efficiency code.

  • P0430Catalytic converter efficiency below the computer's limit (Bank 2)

    Other bank Common general cause

    P0430 is the catalyst efficiency code for bank 2.

All Catalytic converter (catalyst efficiency) codes

P0422 questions

Can I drive with P0422?

KBB says you can for some time, but it is not recommended long-term. A failing converter can clog and cost power.

Is P0422 different from P0420?

Both are catalyst efficiency codes; the checks are the same. P0422 refers to the main catalyst in its definition.

Sources and review

  1. P0422 code: main catalyst efficiency below threshold — Kelley Blue Book (KBB Service Advisor)
  2. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  3. GM PIP5917: direction when P0422 sets on a 2024 6.6L L5P diesel — General Motors / NHTSA
  4. GM 24-NA-058: MIL with P062F, P040B, P041B, P0422 and others (2024 Silverado/Sierra 2500/3500) — General Motors / NHTSA
  5. Audi bulletin 0401: MIL with P0422 and/or P0432 (2000 A6 and A8) — Audi of America / NHTSA
Last reviewed
Written by
Saurabh K.
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