What causes fault code P0422?

• Most likely: Defective three-way catalytic converter (aged or contaminated) • Likely: Exhaust system leak (upstream or near the converter) • Possible: Faulty downstream oxygen sensor (Bank 1, Sensor 2) • Rare: Fuel trim issues (rich/lean conditions) causing catalyst degradation

How do you fix fault code P0422?

STEP 1: Visually inspect the exhaust system for leaks, cracks, or loose connections. Manifold leaks can introduce oxygen and cause false readings. STEP 2: Check for other DTCs (e.g., P0300 misfires or P0171/P0172 fuel trim). These MUST be repaired first as they can damage a replacement catalyst. STEP 3: Perform data logging. At 2500 RPM, the downstream oxygen sensor should show a steady voltage (0.45V - 0.7V). If it oscillates like the upstream sensor, the catalyst is failing. STEP 4: If replacement is necessary, use OEM or high-quality aftermarket converters. Cheap universal cats often fail to clear the code long-term.

Powertrain (Engine & Gearbox)Standard EOBD

P0422 - Main Catalyst Efficiency Below Threshold (Bank 1)

Q: What are the symptoms of fault code P0422?

• Check Engine Light (MIL) illuminated • No noticeable driveability issues (initially) • Failed emissions test (high CO/HC) • Rotten egg smell from exhaust in severe cases

Last updated:1 February 2026

Related:Catalytic Converter, Oxygen Sensor (Bank 1 Sensor 2), Oxygen Sensor (Bank 1 Sensor 1), Exhaust Manifold

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0400

Exhaust Gas Recirculation Flow Malfunction

P0401

Exhaust Gas Recirculation (EGR) Flow Insufficient Detected

P0402

Exhaust Gas Recirculation Flow Excessive Detected

P0403

Exhaust Gas Recirculation (EGR) Control Circuit

Technical Description

The ECU monitors the catalytic converter's efficiency by comparing the signals from the upstream (pre-cat) and downstream (post-cat) oxygen sensors. The upstream sensor should cycle rapidly, while the downstream sensor should remain relatively steady, indicating that oxygen is being consumed in the catalytic process. If the downstream sensor's signal starts to mirror the upstream sensor, it indicates that the catalyst is no longer effectively storing oxygen or reducing emissions (HC, CO, and NOx). P0422 is triggered when the catalyst efficiency drops below the calibrated threshold.

Possible Causes

Defective three-way catalytic converter (aged or contaminated)

Exhaust system leak (upstream or near the converter)

Faulty downstream oxygen sensor (Bank 1, Sensor 2)

Fuel trim issues (rich/lean conditions) causing catalyst degradation

Symptoms

Check Engine Light (MIL) illuminated

No noticeable driveability issues (initially)

Failed emissions test (high CO/HC)

Rotten egg smell from exhaust in severe cases

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P0422 - Main Catalyst Efficiency Below Threshold (Bank 1)

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P0422 - Main Catalyst Efficiency Below Threshold (Bank 1)

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