P0420 Causes and Common Fixes

catalytic converter of the vehicle

The most common reason why fault code P0420 comes up is a problem with the catalytic converter. The function of the catalytic converter is to break down harmful gases created by a gasoline engine. When the upstream and downstream oxygen sensors have identical readings, the catalytic converter is not doing its job of reducing harmful pollutants, and the check engine light is triggered.

This code addresses that there is a problem with the catalytic converter. Bank 1 refers to vehicles with inline four cylinders or the first bank of cylinders on a V6 or V8 engine. Bank 2 refers to the second bank of cylinders on a V6 or V8 engine.

Causes for the P0420 code

  • Damaged or leaking exhaust components
  • Engine misfire
  • Engine oil contamination to the catalytic converter
  • Bad catalytic converter (standard)
  • Bad O2 sensors
  • Bad coolant temperature sensor
  • Damaged or corroded O2 sensor wiring
  • Unplugged or wrongly plugged sensor connectors
  • Damaged sensor connectors
  • Bad or wrong fuel
  • Plugged or leaking fuel injector
  • Out-of-specification fuel pressure.


  • Most noticeably and most commonly, illuminated check engine light
  • Loss of power once the vehicle has reached operating temperature
  • Vehicle derate condition ( not being able to go faster than 40mph)
  • A rotten egg smell coming from the exhaust


  • Repair/replace damaged or leaking exhaust components.
  • Diagnose and repair engine misfires
  • Replace the catalytic converter
  • Repair/replace O2 sensors O2 sensor wiring/connectors.
  • Repair/replace fuel injectors

What does a catalytic converter do?

p0420 due to catalytic converter

The catalytic converter uses fine metals to filter harmful pollutants brought upon by the combustion process. This allows automobiles to run more “green,” expelling less harmful exhaust gases into the atmosphere.

How do I know if my catalytic converter is working?

To track the catalytic converter’s health, two (sometimes more in newer cars) oxygen, or O2 sensors, send readings to the PCM. There is an upstream and a downstream O2 sensor. The upstream O2 sensor readings fluctuate when the vehicle comes to operating temperature and runs in a closed loop.

A closed-loop is when your vehicle has reached and has been running at operating temperature. The PCM sends fuel, and air mixture ratio commands based on the information gathered by sensors on that system vs. OEM default commands. These default air and fuel mixture ratios are used during vehicle startup and are less efficient. This is why many vehicles now have O2 sensor heater circuits to expedite the sensor’s ability to send the PCM correct data.

However, the downstream O2 sensor voltage readings remain constant if your catalytic converter operates as designed.

If the downstream O2 sensor reading fluctuates, which leaves the tasks similar to the upstream sensor, the oxygen levels are too high. So the catalytic converter isn’t reducing the pollutants.


p0420 catalytic
  1. To diagnose a P0420, a technician would hook an OBDII code reader to the vehicle and read codes sent by the PCM.
  2. Use the code reader to see if any other codes contributed to the P0420 fault code. (For example, a code indicating a bad fuel injector)
  3. Follow any leads from the visual inspection (repairing broken or damaged wiring harnesses or connectors, repairing any broken or leaking exhaust components, draining and replacing leaded fuel)
  4. Most commonly, if there was no preexisting condition in the ignition, fuel, or intake system that could damage the catalytic converter, an O2 sensor (either upstream or downstream) has gone bad. A simple replacement of the faulty sensor will often solve your problem. (Changing an O2 sensor without confirming the sensor was bad is the most common mistake when repairing this fault).

How to test an O2 sensor with a multimeter:

  1. Identify which O2 sensor you want to try by following the DTC stored by your vehicle PCM (reading the codes with your OBDII code reader)
  2. Has your multimeter been set up with a back probe lead and a lead with an alligator clip to connect to a known good ground?
  3. Back probe your O2 sensor signal wire and connect your alligator clip to a known good ground (the vehicle negative battery post is the best). Instead of using an alligator clip, back probe the ground wire on your O2 sensor connector.
  4. Start the engine and let it warm up to operating temperature.
  5. Turn your multimeter to the mV DC scale and confirm your readings fluctuate between .10v and .90v (if your sensor is within this spec, your O2 sensor is working correctly). This would complete your test, and no further testing would be required.

If the O2 sensor is not in the specification as listed above, then continue with these steps:

  1. Test the O2 sensor response to a lean fuel consumption event by disconnecting the hose on the PVC valve to allow more air into the engine. This will bring your volt reading to a reading of around .20v (if there is no voltmeter response to the changing condition, that would indicate a bad O2 sensor)
  2. To test for a rich fuel consumption event, reconnect the line to the PVC valve and disconnect the small hose on your air filter assembly.
  3. Block the opening on the hose to restrict airflow to the engine.
  4. Check your volt reading. This reading should be around .08v. (If the sensor did not change due to the changing condition you created, the sensor is not working correctly)


  • Another component could blame if the sensor responded accordingly to the changing conditions and still had a CEL.
  • If the sensor does not respond to the changing conditions, the O2 sensor is not operating correctly and should be replaced.

We hope you find the P0420 Causes and Common Fixes guide helpful. Check these troubleshooting and repair guides for more help on your vehicle.

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