Mitsubishi Outlander: Warm-up catalyst efficiency → below threshold (bank 1)


P0421 It is a generic OBDII fault code. It has exactly the same concept for the Mitsubishi Outlander like all vehicles. It is closely related to P0420 (which is the most common OBD code).

Catalyst efficiency on heating → below threshold (Bank 1)

When it is the only fault code present, P0421 does not usually affect the driving of your Outlander, as it is primarily an emissions-related code.

This code affects “bank 1”, which will be the side of the engine with tube 1 in firing order. If we talk about an inline engine (almost all 4-cylinder engines and some 6-cylinder engines) there is only one bank and you don’t have to worry about which side of the engine has the problem.

P0421 Mitsubishi Outlander

P0421 Symptoms: Mitsubishi Outlander

There are usually not many symptoms associated with P0421. Here are the most common ones.

  • The engine runs poorly when starting
  • Service engine light coming soon
  • Difficult start

Common Causes of Outlander P0421

Mitsubishi Outlander P0421 Diagnostics


A bad catalytic converter can throw this code. In fact, if you find this code without others, it is very possible that the problem is generally the catalyst or the exhaust system.

A visual inspection can be really revealing. If you can safely take a look at the exhaust system, do so.

Here is a great product on diagnosing a catalyst from Advance Coche Parts.


If your Outlander has or is fouling its spark plugs, it may be more visible when starting. By the time the engine and spark plugs are warmed up, everything remains the same. Other ignition-related items can also cause the code (such as spark plug wires, coils, etc.).

O2 sensors

If the oxygen sensors stop reading correctly, it may issue code P0421. A bad O2 sensor is one of the most common causes of this code.

The wiring that runs to and from the oxygen sensors is much more susceptible to damage than most other parts of your Outlander’s wiring harness. This is caused by its proximity to the hot exhaust, as well as its location under the vehicle.

Here is a great video on how to diagnose an imperfect oxygen sensor from Ratchets and Wrenches.

If you need a new oxygen sensor, they are relatively inexpensive.


Although an imperfect catalyst is the most common cause of P0421 generally, it is a good idea to examine and test the oxygen sensors and ignition system before committing to that expensive repair for your Outlander.

Good luck with the diagnosis. If there is anything you want to add, leave a comment now.


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