The error code P0422 tells us that the Bank 1 Main Catalytic Converter’s emissions efficiency has fallen below the minimum allowable limit. This threshold is traced by a Catalyst Monitoring Oxygen Sensor positioned on or near the outlet of the Catalytic Converter.

A vehicles’ catalytic converter (catalyst) helps improve tailpipe emissions by oxygenating the exhaust gasses. It helps convert hydrocarbons (HC), Carbon Monoxide (CO), and oxides of Nitrogen (NOx) into water vapor (H20), Carbon Dioxide (CO2), and Nitrogen (N).

The PCM (Powertrain control module) monitors the catalyst efficiency by means of a post-catalyst oxygen sensor. This sensor is used primarily for this purpose. It should switch much slower than the front oxygen sensors. If at any point the PCM detects that the post-catalyst O2 sensor is switching too rapidly (indicating no change between the inlet and outlet of the catalytic converter) it interprets this as a catalyst that isn’t functioning properly.

As the code description implies the P0422 code means that the vehicle’s control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0422 code is combined with other codes, try fixing the other codes first.


The Engine Control Module or ECM monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).

A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.

When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

Common Symptoms

The following are only some of the symptoms of a P0422 DTC:

  • Increased tailpipe emissions
  • MIL (Malfunction Indicator Lamp) illumination
  • Service Engine Soon Warning Light

Possible Causes

Potential causes of a P0422 code include:

  • Bad or non-functional catalyst
  • Engine operating out of optimal air:fuel ratio
  • Defective Catalytic Converter
  • Engine misfires have damaged the Catalytic Converter
  • Internal engine damage resulting in high oil consumption and/or a leaking Head Gasket has damaged the Catalytic Converter
  • Warm Up Three Way Catalytic converter deterioration or malfunction
  • Exhaust system leaks
  • Loose front or rear HO2S
  • Faulty front heater O2 sensor
  • O2 sensors harness is open or shorted
  • O2 sensors poor electrical connection

How to Check

The P0422 code is set when the Engine Control Module otherwise known as ECM detects rear HO2S value difference becomes closer to value of front HO2S. The ECM checks when the vehicle speed is 25-60 MPH. Also, when the load is 22-45 percent and engine speed is 1000-3000 RPM.

How to Fix

Catalytic converters contain precious metals like platinum, palladium, and rhodium among others. Basically, the cheaper the part, the less precious metals are used. It should be noted that there are some automobile makes that are very finicky when it comes to catalytic converters and the logic that the PCM uses to monitor them. Just because the parts guy says it will work doesn’t mean that it will. The reason is that the engineering of the vehicle was done with high quality components in mind and it can’t be replaced with a low cost catalyst.

There are also different kinds of catalytic converters. Some makes will allow for a cheaper catalytic converters while others simply will not. Toyotas & Subarus are the most notorious for this. Subarus require CARB certified catalysts to keep the check engine light off and won’t tolerate an aftermarket catalyst, no matter how expensive. So basically if you have a recurrence of this code after replacing the catalytic converter with an aftermarket part, this may be your problem. If you have this code and haven’t replaced the catalytic converter, there’s basically two likely causes: The catalyst is no good, or the engine is running out of optimal air:fuel ratio. A tune up or engine repair may correct it. Otherwise you’ll need to replace the catalyst.