P0422 – Main catalytic converter, bank 1 -efficiency below threshold

Daniel Young

By Daniel Young (Contact Me)
Last Updated 2016-11-18
I am an ASE certified Master Automotive Technician with 32 years of experience. Electronics, Cars, Trucks, audio, ATV's, motorcycles, washers, driers. ect. ect. Hell, "If I can't fix it, it's not broken!"

Trouble CodeFault LocationProbable Cause
P0422 Main catalytic converter, bank 1 -efficiency below threshold Catalytic converter, wiring, HO2S 2

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What Does Code P0422 Mean?

The powertrain control module (PCM) is the computer that controls everything to do with the engine and it is receiving information that the catalytic converter on bank 1 is not working as well as it should.

There are three things you will need to know in order to understand what this code really means:

  • The purpose of a three-way catalytic converter(TWC)
  • What an oxygen sensor(O2S) is
  • How PCM monitors catalyst operation
  1. Exhaust gasses from automobile exhaust, in general, are harmful to the environment. Three gases in particular are the most harmful; Carbon Monoxide(CO), Hydrocarbons(HC), and Oxides of Nitrogen(NOX). The purpose of a TWC is to eliminate as much of these three gases as possible. When exhaust gas leaves the engine it is extremely hot as it enters TWC. When these gasses and oxygen come in contact with the platinum catalyst they are reduced to carbon dioxide (co2) , water(H2O) and nitrogen oxide(NOS). In this process the amount of oxygen is also reduced.
  2. An oxygen sensor is a ceramic sensor located in the exhaust pipe both before, upstream, and after, downstream, of the catalytic converter. The sole purpose of these sensors is to measure the amount of oxygen in the exhaust gasses and send this information to the PCM. On four cylinder engines there are generally two oxygen sensors; Bank 1 sensor 1 and bank 1 sensor 2. On v6 and v8 engines there is an additional two sensors; bank 2 sensor 1 and bank 2 sensor 2. In every application, regardless of bank number, sensor 1 is always upstream of the TWC and sensor 2 is always downstream.
  3. The PCM uses the oxygen sensor information to determine the amount of oxygen entering and leaving the catalyst. The amount leaving should be significantly less than amount entering; that difference is a direct reflection of how well, or efficient, the catalyst is performing. By combining the input of other engine sensors and the amount of fuel being injected the PCM expects to see a predetermined amount of difference in the oxygen content. When that difference falls below the predetermined amount the PCM turns on the check engine light and sets P0422 in memory

Armed with this knowledge let’s take a look at some causes, some quick tests and some possible solutions for diagnostic trouble code P0422.

What are the common causes of code P0422 ?

  • Failing Catalytic converters are the number one cause of catalyst efficiency codes. Whether due to high mileage or damage catalyst material from contaminated fuel more than half the time the catalyst itself is to blame.
  • Engine thermostat
  • Faulty oxygen sensor(S) and/or wiring
  • Faulty PCM (RARELY)

What are the symptoms of code P0422 ?

If this is the only code present there will most likely be very little difference in the way the engine runs. The gas mileage will go down and the check engine light will be on.

How do you troubleshoot code P0422 ?

One of the simplest ways to determine if a catalyst is operating is to run the engine at 2500 rpm for two and a half minutes and then measure the temperature at the input and output of the catalytic converter. The output should be a minimum of 200 degrees Fahrenheit hotter than them input. If there is little or no difference suspect the catalyst. If there is a significant difference the two then suspect a faulty downstream oxygen sensor. Either a pyrometer attachment for a digital volt meter or a laser type temperature probe will work to perform this test.

Everything in an automotive drivetrain is designed to be at its most efficient when at operating temperature. Catalytic converters are no exception to this rule. Average operating temperature of modern cars is 220 degrees Fahrenheit and is more critical than most people realize. Ask a Parts store what the OEM thermostat temperature is for your specific make and model and if it’s not running up to that temperature change out your thermostat and recheck system.

If these steps prove to be inconclusive you may need to seek the help of a professional before replacing the PCM. Replacing The PCM can be costly and will most likely require programming of the new part before the vehicle will run. You don’t want to find yourself with a non-running vehicle after installing an expensive PCM.

Codes Related to P0422

Trouble codes P0420, and P0421 are essentially the same as P0422 in that they all point to the catalyst not working properly.

Any codes for oxygen sensors on bank 1 such as P0141 or P0135 should be diagnosed first because they could possibly be the cause of P0422.

BAT Team Discussions for P0422

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