Mazda 6: Warm-up catalyst efficiency → below threshold (bank 1)


P0421 It is a generic OBDII fault code. It has the same concept for Mazda 6 like any other vehicle. It is closely related to P0420 (which is the most common OBD code).

Catalyst heating efficiency → below threshold (Bank 1)

While it is the only fault code present, P0421 is not likely to affect the drivability of your 6, as it is primarily an emissions-related code.

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

P0421 Mazda 6

P0421 Symptoms: Mazda 6

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

  • The engine runs poorly when starting
  • The service engine soon light
  • Difficult start

Common 6 P0421 Causes

Mazda 6 P0421 Diagnostics

Catalytic converter

A catalytic converter in poor condition can throw this code. In fact, if you find this code without others, it is very likely that the problem is usually the catalyst or the exhaust system.

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

Here is a great product on catalytic converter diagnosis from Advance Auto Parts.


If your 6 has dirty or staining spark plugs, it may be much more obvious when starting. Once the engine and spark plugs are warmed up, everything stays the same. Other ignition-related components can also cause the code (such as spark plug wires, coils, etc.).

O2 sensors

If the oxygen sensors stop reading properly, 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 of the other parts of your 6’s wiring harness. This is due to its proximity to the hot exhaust, as well as its position low the vehicle.

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

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


Although a faulty catalyst is the most common cause of P0421 generally, it is a great idea to examine and check the Oxygen Sensors and Ignition System before committing to that expensive repair for your 6

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


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