Aged or failed catalytic converter that can no longer store oxygen efficiently; Exhaust leaks upstream of or at the converter, allowing air to skew sensor readings; A lazy or contaminated downstream oxygen sensor that responds too slowly; An upstream oxygen sensor that is biased rich or slow, causing the converter to work improperly; Engine misfires or rich-running conditions that have overheated or poisoned the catalyst; Oil or coolant consumption entering the exhaust stream and coating the catalyst; A restricted exhaust or damaged converter substrate from impact or thermal shock

DTC codes / P04xx · Auxiliary emission controls / P0422

P0422Main Catalyst Efficiency Below Threshold (Bank 1)

Q: What does code P0422 mean?

Main Catalyst Efficiency Below Threshold (Bank 1). P0422 means the powertrain control module (PCM) has determined that the upstream and downstream oxygen sensors are reporting too similar a signal across the bank 1 catalytic converter, indicating the converter is no longer storing and releasing oxygen effectively. The PCM runs a monitor under specific closed-loop conditions and compares the switching activity of the two sensors; when the downstream sensor mirrors the upstream sensor too closely, the efficiency threshold is exceeded and the code is stored.

P0422 means the powertrain control module (PCM) has determined that the upstream and downstream oxygen sensors are reporting too similar a signal across the bank 1 catalytic converter, indicating the converter is no longer storing and releasing oxygen effectively. The PCM runs a monitor under specific closed-loop conditions and compares the switching activity of the two sensors; when the downstream sensor mirrors the upstream sensor too closely, the efficiency threshold is exceeded and the code is stored.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaAuxiliary emission controls

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Slight decrease in fuel economy as the PCM richens the mixture to compensate
A faint sulfur or rotten-egg smell from the exhaust in some cases
Failed emissions test due to high hydrocarbon (HC) or carbon monoxide (CO) readings
Occasional rough idle or hesitation if the converter is severely degraded
No drivability complaints at all in many instances, especially on newer vehicles

Common causes, most likely first

Aged or failed catalytic converter that can no longer store oxygen efficiently
Exhaust leaks upstream of or at the converter, allowing air to skew sensor readings
A lazy or contaminated downstream oxygen sensor that responds too slowly
An upstream oxygen sensor that is biased rich or slow, causing the converter to work improperly
Engine misfires or rich-running conditions that have overheated or poisoned the catalyst
Oil or coolant consumption entering the exhaust stream and coating the catalyst
A restricted exhaust or damaged converter substrate from impact or thermal shock

How to diagnose and fix it

Inspect the entire exhaust system for leaks, loose clamps, and damaged gaskets before condemning the converter
Use a scan tool to view live upstream and downstream oxygen sensor data and confirm the downstream sensor is switching similarly to the upstream sensor
Check for stored misfire, fuel trim, or oxygen sensor codes that could indicate an underlying engine problem damaging the converter
Perform a backpressure test or inspect the converter substrate with a borescope to rule out a restricted or melted catalyst
Verify the downstream oxygen sensor responds correctly to a propane enrichment test and replace it if sluggish or contaminated
Repair any engine performance faults (misfire, rich condition, coolant or oil consumption) before installing a new converter
Replace the catalytic converter only after all upstream causes have been ruled out and the monitor still fails

Before you replace the catalytic converter

A P0422 is frequently set by a failing downstream oxygen sensor or a small exhaust leak rather than by the converter itself, so always verify sensor switching and leak-test the exhaust first.
Engine misfires, a stuck-open thermostat, or excessive oil consumption can poison or overheat the catalyst and set this code; fixing the converter without repairing the root cause will destroy the new unit.
Some vehicles require a specific drive cycle to complete the catalyst monitor, and the code may reappear only after several trips even if the repair was successful.
A converter that is only marginally efficient may pass the monitor in warm weather but fail in cold weather, so diagnose the fault under the same conditions that set the code.
Aftermarket converters with lower precious-metal loading can trigger P0422 again quickly on vehicles that are sensitive to catalyst efficiency, so choose a quality replacement matched to the application.

Can I keep driving with P0422?

The vehicle is generally safe to drive with P0422 because the fault does not cause an immediate safety risk, but continued driving with a degraded converter can lead to higher emissions and eventual damage to other exhaust components. Avoid ignoring the light for long periods, and do not tow heavy loads or drive at sustained high speeds until the fault is diagnosed, as extra heat can worsen catalyst damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.