Mazda CX-3: Warm Up Catalyst Efficiency → Below Threshold (Bank 1)

P0421 is a generic OBDII trouble code.  It has the same meaning for the Mazda CX-3 as it would any other vehicle.  It is closely related to P0420 (which is the most popular OBD code).

Warm Up Catalyst Efficiency → Below Threshold (Bank 1)

When it’s the only trouble code present, P0421 will typically not affect the drivability of your CX-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 the firing order.  If it’s an inline engine (almost all 4 cylinders and some 6 cylinder motors) there’s only one bank and you don’t need to worry about which side of the engine has the problem.

P0421 Mazda CX-3


P0421 Symptoms:  Mazda CX-3

There typically aren’t many symptoms associated with P0421.  Here are the most common ones.

  • Rough running engine at startup
  • Service engine soon light
  • Hard starting


Common CX-3 P0421 Causes

Mazda CX-3 P0421 Diagnosis

Catalytic Converter

A bad catalytic converter can throw this code.  In fact, if you find this code with no other codes, it is highly likely the Cat or the exhaust system in general is the problem.

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

Here’s a great article on diagnosing a catalytic converter from Advance Auto Parts.


Spark Plugs

If your CX-3 has fouled or is fouling plugs, it may be most apparent at start up.  Once the engine and plugs warm up its business as usual.  Other ignition related components can cause the code as well (such as plug wires, coils, etc…).


O2 Sensors

If the Oxygen sensors are no longer reading properly, it can throw the P0421 code.  A bad O2 sensor is one of the most common cause of this code.

The wiring going to and from oxygen sensors is more susceptible to damage than most other parts of your CX-3’s wiring harness.  This is due to its proximity to hot exhaust, as well as its position under the vehicle.

Here’s a great video on how to diagnose a bad oxygen sensor from Ratchets and Wrenches.

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



While a bad catalytic converter is the most common cause of P0421 in general, it’s a good idea to inspect and test the Oxygen Sensors and Ignition System before committing to that expensive repair for your CX-3. 

Good luck diagnosing.  If there is anything you’d like to add, please leave a comment below.