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Mazda 3: Warm-up catalyst efficiency → below threshold (bank 1)


P0421 It is a generic OBDII fault code. It has exactly the same concept for the Mazda 3 like any other vehicle. It is closely related to P0420 (which is the most popular OBD code).

Catalyst heating efficiency → below threshold (Bank 1)

While it is the only fault code present, P0421 usually does not affect the drivability of your 3, as it is primarily an emissions-related code.

This code affects “bank 1”, which will 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 don’t have to worry about which side of the engine has the problem.

P0421 Mazda 3

P0421 Symptoms: Mazda 3

Normally there are 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

Recurrent causes of 3 P0421

Mazda 3 P0421 Diagnostics

Catalytic converter

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 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 article on diagnosing a catalyst from Advance Coche Parts.


If your 3 has fouled or is fouling its spark plugs, it may be more obvious when starting. Once 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 throw the P0421 code. A bad O2 sensor is one of the most common causes of this code.

The wiring leading to and from the oxygen sensors is more susceptible to damage than most other parts of your 3’s wiring harness. This is due to its proximity to the burning exhaust, as well as its position under 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 an imperfect catalyst is the most common cause of P0421 generally, it is a great idea to inspect and check the Oxygen Sensors and Ignition System before committing to that expensive repair for your 3

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


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