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Severity: Low Code type: Generic (all makes)

P0422 Code: Main Catalyst Efficiency Below Threshold (Bank 1)

The main underfloor catalytic converter on bank 1 no longer stores enough oxygen to pass the ECU's test.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
Main Catalyst Efficiency Below Threshold (Bank 1)
System
Emission Controls · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$80–$2,500
DIY difficulty
Moderate

Some exhaust systems use two converters in series on each bank: a small warm-up catalyst close to the engine and a larger main catalyst under the floor. Where the ECU monitors them separately, it uses an oxygen sensor between the two and another behind the main converter. P0422 means the main catalyst on bank 1 failed that test: the sensor behind it starts copying the swings of the sensor in front of it, a sign the converter can't store and release oxygen any more. Most vehicles report converter problems as P0420, so P0422 appears only on systems whose software splits out the main catalyst. The underfloor unit usually dies from age, heat damage after misfires or rich running, or contamination by oil or coolant, and exhaust leaks can fake the result.

Can I keep driving? The car is normally fine to drive. If you notice a rattle under the floor, a rotten-egg smell, loss of power at highway speed or a flashing check engine light, get it checked soon, since a broken or clogged converter can restrict the exhaust.

How to read P0422

How to read P0422P10243224
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 4 Auxiliary emission controls
  4. 22 Specific fault

Symptoms

  • Check engine light with no change in how the car drives
  • Rotten-egg (sulfur) smell after hard acceleration
  • Rattle from under the middle of the car when tapping the converter shell
  • Loss of top-end power if the substrate has broken up and is blocking flow
  • Fails an emissions test on readiness or tailpipe readings

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Main underfloor catalyst aged or thermally damaged 32%
  2. Exhaust leak between the middle sensor and the main converter, often at the flex pipe or a gasket 18%
  3. Sensor behind the main converter lazy, contaminated or wired wrong 15%
  4. Earlier misfires or rich running overheated the converter, and the cause may still be present 13%
  5. Oil burning or coolant leak poisoning the catalyst 12%
  6. Replacement converter that isn't approved or sized for the vehicle 10%

Diagnosis step by step

Diagnostic flow: P04221Fix engine problems first2Compare middle and rear sensorsignals3Check Mode 6 catalyst results4Hunt for exhaust leaks5Tap and inspect the converter6Replace with a legal, correct unit

  1. Fix engine problems first

    Check for misfire, fuel trim, oxygen sensor or thermostat codes and deal with those before judging the converter. Long-term fuel trims should sit roughly within ±10%; a lean or rich engine can make a good converter fail its test.

    Tools: OBD-II scanner

  2. Compare middle and rear sensor signals

    Warm the engine fully and hold about 2,500 rpm. The sensor between the converters may still move somewhat, but the one behind a healthy main catalyst should stay fairly steady, typically around 0.6 to 0.8 V. If the rear trace mirrors the middle one, the main converter is worn out.

    Tools: OBD-II scanner

  3. Check Mode 6 catalyst results

    Look up the catalyst monitor test results in Mode 6. A value near or past the limit on the main catalyst test confirms the ECU's decision and lets you check the repair later without waiting for the light.

    Tools: OBD-II scanner

  4. Hunt for exhaust leaks

    With the engine cold, listen and feel for puffs at the flex pipe, flanges and sensor bungs between the two converters and behind the main unit. A smoke tester fed into the tailpipe makes small leaks obvious. Air drawn in there fools the rear sensor.

    Tools: Smoke machine, Flashlight

  5. Tap and inspect the converter

    Tap the main converter shell with a rubber mallet: a rattle means broken substrate. A dented or discolored shell points to impact or overheating. If power is down at speed, check exhaust backpressure before condemning anything else.

    Tools: Gloves, Flashlight

  6. Replace with a legal, correct unit

    If the converter is confirmed bad, fit a direct-fit unit certified for your vehicle; in California and states following its rules, it must be CARB-approved. Clear codes and confirm the catalyst monitor passes before any inspection.

    Tools: OBD-II scanner, O2 sensor socket, Torque wrench

Where the parts are

Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2) – P04221212345
Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2)
  1. 1Upstream O2 sensor
  2. 2Catalytic converter
  3. 3Downstream O2 sensor
  4. 4Diesel particulate filter
  5. 5Turbocharger

Repair options & costs

RepairDIY?Estimated cost
Repair exhaust leak or flex pipe No $80–$350
Replace rear oxygen sensor Yes $120–$350
Replace main catalytic converter (EPA-compliant) No $500–$1,500
Replace main catalytic converter (CARB-approved) No $900–$2,500
Fix oil burning or coolant leak causing contamination No $300–$2,500

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Emissions / smog test impact

Will fail

P0422 keeps the check engine light on and the catalyst monitor failed, so the vehicle fails an OBD-II smog check. Replacement converters must be legal for your state; in California and states that adopt its rules, only CARB-approved units pass.

Frequently asked questions

How is P0422 different from P0420 and P0421?

P0420 covers the bank 1 catalyst system as a whole and is what most vehicles use. P0421 points at the small warm-up converter near the manifold. P0422 singles out the larger main converter further back, on systems that monitor it separately.

Do I need to replace both converters?

Not usually. The code isolates the main catalyst, so the warm-up converter can stay if it passes its own test. Replace only what the monitor has condemned, after fixing the cause of the failure.

Can a new oxygen sensor fix P0422?

Only if the rear sensor is actually faulty, for example slow, contaminated or with a broken heater. If the rear sensor mirrors the middle one and responds quickly to a snap throttle, the sensor is doing its job and the converter is the problem.

Will a cheap universal converter clear it?

Often not for long. Low-cost units may carry too little catalyst material for the ECU's test, and they may not be legal in your state. A direct-fit certified unit is the safer choice.

Sources