| |

Honda Shuttle: Heating Catalyst Efficiency → Below Threshold (Bank 1)


P0421 It is a generic OBDII fault code. It has exactly the same meaning for the Honda Shuttle as any other vehicle. It is closely related to P0420 (which is the most common OBD code).

Catalyst efficiency on heating → below threshold (Bank 1)

While it is the only fault code present, P0421 usually does not affect the drivability of your shuttle, 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 it is an inline 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 Honda Shuttle

P0421 Symptoms: Honda Shuttle

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

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

Common Causes of P0421 Shuttle

Honda P0421 Shuttle Diagnostic

Catalytic converter

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

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

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


If your shuttle has or is fouling the spark plugs, it may be more visible when starting. Once the engine and spark plugs are warmed up, everything stays 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 properly, it may issue the P0421 code. A bad O2 sensor is one of the most common causes of this code.

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

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

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


While a faulty catalyst is the most common cause of P0421 in general, it is a good idea to inspect and check the oxygen sensors and ignition system before committing to that expensive repair for your Shuttle.

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


Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *