1. Autos

Your suggestion is on its way!

An email with a link to:


was emailed to:

Thanks for sharing About.com with others!

Questions and Answers

Honda Civic EX DTC P0420

Q. Vincent, I have got an error code and really dont know how to troubleshoot it. The error code and the explanation are vague at best. The code is P0420 Catalyst System Efficiency Below Threshold (bank1). I have reset the check engine light and seem to receive the error when traveling at speeds above 65 mph for a period of time.

Honda Civic EX DTC P0420

My question is, does that mean the Oxygen Sensor or Catalytic Converter happen to be the problem? I have two sensors one before the Catalytic Converter and one after the Catalytic Converter and not to mention that the cost of those two are a small fortune to obtain never the less I still have to solve the problem. If you can help it would be greatly appreciated.

  • 2000 Honda Civic EX
  • 1.6 liter D16Y8
  • Automatic transmission
  • 67,000 miles
  • P/S, A/C, Cruise control

Thanks In advance,

A. The catalyst monitor test is performed once per drive cycle if all test conditions are met. The main enable criteria to enable a catalyst test are: a steady Throttle Position Sensor (TPS) reading and calculated temperature of catalytic converter, using Engine Coolant Temperature (ECT) sensor, Intake Air temperature (IAT) sensor, Vehicle Speed Sensor (VSS) load and run time, indicating system is fully warmed up.

Once the enable criteria are met, 50 samples of the sensor voltage amplitudes are collected. The results of the 50 samples are averaged and converted to a millivolt (Mv) reading which indicates the efficiency of the converter.

The catalyst monitor tests consist of two stages. Stage 1 voltage ranges from -555 Mv to a positive 550 Mv. When this value goes above 0 Mv indicating stage one has failed, the stage 2 test will start to be used in determining the efficiency.

When stage 2 reaches the fail threshold the Check Engine Light (MIL) will be illuminated and a DTC P0420 will be set. The Scan tool can display the current value in millivolts for stage 1 as well as indicate if the stage 1 test has passed or failed.

Whenever codes are cleared or the battery has been disconnected the results of stage 1 will be reset to a -555 Mv. For more information on the catalyst test refer to General Description of the Catalyst Monitor System.

The stage 1 will fail when the catalyst efficiency becomes less than 75% efficient. After stage one fails, the 50 sample test results are used by stage 2. When the results of stage 2 are greater than the fail threshold Mv, catalyst efficiency below about 70%, the MIL will be illuminated and a DTC P0420 will be set.

Probable causes of a DTC P0420 are:

  • Exhaust system leaks between the front and rear sensors or rear sensor leaking.
  • Catalyst deteriorated due to excessive time operating vehicle with a catalyst damaging misfire.
  • Contamination of converter due to incorrect sealant, fuel, or fuel additive.

NOTE: A catalyst monitor test will not be performed if a fault exists in the rear or front oxygen sensor circuits.

DTC P0420, once set, should remain active. However, under certain driving conditions the catalyst monitor test may pass. This condition may occur if driving the vehicle at highway speeds (high air flow) but if the P0420 is set and no exhaust leaks are found the catalytic converter should be replaced.

If a misfire (P0300) is detected P0420 test will not run. Be sure to correct a misfire condition before replacing the converter.

Additional Information provided courtesy of AllDATA

Back to Index
© 2005 Vincent T. Ciulla

©2017 About.com. All rights reserved.