P0420 / P0421 (year dependent) – Catalytic Converter System (Mazda)

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By Randy Worner (Contact Me)
Last Updated 2023-11-21
ASE/ACA World Class Technician
CodeFault LocationProbable Cause
P0420 Catalytic Converter System
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Catalytic converter, wiring, HO2S 2

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Table of Contents

  1. What Does Code P0420 Mean?
  2. Where is the P0420 sensor located?
  3. What are the common causes of code P0420?
  4. Get Help with P0420

What Does Code P0420 Mean?

This generic powertrain/engine diagnostic trouble code typically applies to most manufacturers since 1996. P0420 applies to those models built up to around 2008. After that, P0421 is utilized by Mazda.

This code is focused on the voltage being returned by the Air Fuel Ratio Sensor (AFR) and the Post Catalytic Converter Oxygen Sensor (O2S – downstream) to the engine controller (ECM/PCM). The PCM monitors the voltage and the sensor’s ability to send both a low voltage (typically below .5V) and a high voltage (above .5V). When the engine is started cold, both O2 sensors should produce the same voltage. Once the engine warms up and more importantly, the catalytic converter, the upstream O2 sensor should switch above and below 0 amperes. The downstream O2 sensor voltage should begin to become more stable, less fluctuating. This stable downstream O2 sensor voltage is due to the catalytic converter slowly releasing oxygen into the exhaust stream as it is being utilized. The downstream O2 sensor voltage should not switch as fast. It will look as if it is stuck around one set voltage. If the voltage on the downstream O2 sensor mirrors the upstream O2 sensor, or the voltage varies too much, a fault code is stored. This may be a mechanical issue (failed catalytic converter) or an electrical fault (sensor/signal issue). Always remember, check the engine for any type of misfire, as this will lead to premature catalytic converter failure.

Where is the P0420 sensor located?

On most Mazda 4-cylinder engines the Oxygen Sensor is screwed into the front exhaust manifold. On V6 engines, the Oxygen Sensor is screwed into the exhaust manifold that attaches to the head that has the number cylinder 1 in it. In all cases, the Oxygen sensor that is responsible for this code will be in the exhaust system before the first catalytic converter.

Typically, the causes for this code to set are:

Most common:

  • Failed Catalytic Converter
  • Oxygen sensor – Downstream O2 sensor signal circuit voltage shorted to power or ground, failed O2 sensor
  • Failed Upstream AFR Sensor / exhaust leaks close to sensor

Contributing Factors

  • Fuel system components – including injectors and fuel quality
  • Worn ignition system components, to include the coil, plug wires, spark plugs, etc.
  • (Misfires allows the unburned fuel and air to escape to the exhaust system. Misfires cause O2 voltage to stay low due to high oxygen content coming out of the cylinder.)

Last possibility

Failed PCM – very rarely

What are the common causes of code P0420?

Specific to Mazda vehicles, these are the more common issues with the Catalyst System Efficiency Below Threshold (Bank 1) / Catalytic Converter System fault code:

  • Mazda 2 series w/1.5L – Typical issues with these vehicles is failed catalytic converters, AFR sensors and failed Post Catalytic Converter Heated Oxygen sensors (HO2S). After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda 3 series w/2.0L/2.5L – Check for TSB to update PCM if the code is present but no engine performance issues are noted. Typical issue is the Catalytic Converter. Also note spark plug/spark plug boot issues for misfire cause. Visually inspect spark plugs and boots; replace if unsure. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda 5 series w/2.5L – noted spark plug/spark plug boot/ignition coil issues for misfire cause, as these cause premature catalytic converter failure. Visually inspect spark plugs and boots; replace if unsure. Check ignition coil primary and secondary resistances. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda 6 series w/2.5L Skyactiv – Check for TSB to update PCM if the code is present but no engine performance issues are noted. There are also noted spark plug issues for misfire cause. Ensure only OEM spark plugs are utilized. Off brand/Non-OEM spark plugs can cause misfires. After that, basic fault code diagnosis/misfire diagnosis.
  • If engine in the Mazda 6 is a V6, check out known issues for the CX9 w/V6 engine.
  • Mazda CX5 series w/2.0L/2.5L Skyactive– Check for TSB to update PCM if the code is present but no engine performance issues are noted. Check ignition coil primary and secondary resistances. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda CX7 series w/2.5L – Typical issue is the Catalytic Converter. Also note spark plug/spark plug boot/ignition coil issues for misfire, as these cause premature catalytic converter failure. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda CX9 series w/3.7L V6 – Typical issue is the Catalytic Converter. Please note injector/ignition coil issues, as these cause premature catalytic converter failure. Check injector resistances between cylinders. Check ignition coil primary and secondary resistances. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda MX5 series w/2.0L – Typical issues with these vehicles is failed catalytic converters, AFR sensors and failed Post Catalytic Converter Heated Oxygen sensors (HO2S). Please note spark plug/spark plug boot/ignition coil issues as these misfires can cause premature catalytic converter failure. After that, basic fault code diagnosis/misfire diagnosis.
  • Mazda Tribute w/2.3L/2.5L 4 cylinders and 3.0L V6 – please see Ford Escape

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