P0420 – Catalytic converter system, bank 1 -efficiency below threshold


Trouble CodeFault LocationProbable Cause
P0420 Catalytic converter system, bank 1 -efficiency below threshold Catalytic converter, wiring, HO2S 2

What Does P0420 Mean?

The P0420 and the P0430 codes are caused by catalyst inefficiency.  Replacing your catalytic converter is costly, so, knowing if a new catalytic converter is needed pays off.

Your car’s exhaust system (below) comprises: a catalytic converter (s), at least two oxygen sensors and exhaust pipes that route the engine’s exhaust from the engine, through the catalytic converter, and out the car.  The oxygen sensors sample your car’s exhaust before and after the catalytic converter.  In response to the amount of oxygen found in the exhaust, the oxygen sensors output a voltage which can be read with a multi-meter, oscilloscope, or scan tool.  The oxygen sensor upstream of the catalytic converter will normally be referred to as Bank 1 O2 Sensor 1 (B1O2S1) and the downstream oxygen sensor will be Bank 1 O2 Sensor 2 (B1O2S2).

O2 Sensor Bank 1 Bank 2

Oxygen Sensor

Zirconia oxygen sensors are the most common type of sensor found in cars.  The sensor measures the difference in oxygen present in atmospheric air (outside air in the figure below) and the oxygen present in the exhaust gas.  The percentage of oxygen in the atmosphere is 21% and the exhaust gas oxygen can vary between 0% (rich condition) and 2% (lean condition).  The oxygen sensor’s voltage output, which is read by the car’s computer, will decrease in a lean condition and increase in a rich condition.

Oxygen Sensor

A normal reading for an oxygen sensor will fluctuate sinusodially between 900 millivolts (rich condition) and 100 millivolts (lean condition).  Why does the amount of oxygen vary if the car is functioning properly?  Doesn’t the computer always maintain a constant stoic metric mixture of fuel and air?  There are two reasons: (1) catalytic converter efficiency increases when the exhaust gas mixture is either slightly rich or slightly lean so the car’s computer purposely varies the combustion mixture to increase and decrease the oxygen remaining post combustion (2) the oxygen sensor acts like a switch and is either “on” or “off” in response to very small changes in the air fuel ratio.

O2 Sensor Voltage Graph

Oxygen sensor waveforms can exhibit “hash” or high frequency oscillations which are much more rapid than rich lean oscillations.  It is important to identify the difference between hash caused by misfires and signal noise.  Team ZR-1 has an excellent post on “hash”: http://www.teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1458

Catalytic Converter

The catalytic converter changes the chemical composition of the exhaust gas mixture.  A good catalytic converter will cause the oxygen in the exhaust to react with any hydrocarbons creating water and carbon dioxide.  The important thing to remember for the P0420 trouble code is that there should be no oxygen remaining in the exhaust mixture downstream of the catalytic converter.

Troubleshooting P0420

A functioning exhaust system should have a precatalytic converter oxygen sensor reading that varies sinusodially between 100 millivolts and 900 millivolts and a downstream oxygen sensor (oxygen sensor after the converter- see below) reading which is constant above 600 millivolts.  If your oxygen sensors show this behavior then the catalytic converter is efficiently converting all the oxygen in the exhaust.  The constant high voltage from the oxygen sensor after the converter tells us there is no oxygen post catalytic converter.

Oxygen Sensors & Cat

How do I know that my catalyst is bad? Will a bad oxygen sensor or exhaust leaks trigger the P0420 code?  If both your oxygen sensors show identical sinusodial readings, it is very unlikely anything other than a bad catalyst is the cause.  No combination of exhaust leaks can duplicate downstream and upstream sinusoidal oxygen sensor readings.  Described below is a propane test which will verify your catalytic converter is bad.

IMPORTANT: Remember that when testing oxygen sensors the car must be warmed up.  You will not get any oxygen sensor readings if you have just started your car…

Troubleshooting Exhaust Systems with Air-Fuel Sensors

Many newer cars do not have traditional oxygen sensors.  Instead the car industry has transitioned to air-fuel sensors or wideband oxygen sensors.  One of the first manufacturers to make the change was Toyota.  Since 2006, Toyota’s cars have wideband oxygen sensors.

Air-fuel sensors or wideband oxygen sensors are significantly different from traditional zirconia oxygen sensors.  The output of an air-fuel sensor will be a constant value – gone are the varying sine waves of the oxygen sensor.  Another difference is the air-fuel sensor’s response to fuel.   An air-fuel sensor will decrease its output voltage for a rich condition and increase its output for a lean condition.  This is the opposite of zirconia oxygen sensors!

The strange behaviour of these air-fuel sensors is caused by the underlying sampling of the air fuel mixture.  Whereas, the traditional oxygen sensor could only detect a rich or lean mixture and not the degree of richness or leanness, a wideband oxygen sensor calculates the air fuel ratio with precision.  The wideband sensor output for a 15:1 air fuel ratio will be different than the output for a 20:1 air fuel ratio.  With traditional zirconia oxygen sensors both these ratios would be lean and the resulting output would be the same – a low voltage of 100 millivolts.

If you would like to know more about the physical differences between the two sensors, Clemson University has an excellent article with figures including Nernst and Pump cell structures: http://www.cvel.clemson.edu/auto/sensors/oxygen.html

Catalytic Converter Propane Test

Introducing propane into the car air intake will aid diagnosing the catalytic converter and O2 sensors.  To run this test the car should be warmed up and running in a closed loop.

1.  Connect the scan tool

2.  Set the scan tool to monitor the upstream and downstream O2 sensors and the short and long term fuel trims

3. With the car running, introduce propane into the air intake (all your  O2 sensors should respond with a high steady voltage)

Oxygen Sensor with Propane Response

4.  Keep adding propane until the short term fuel trim is over -20%

5.  Once a rich condition is reached (step 4), stop adding propane

6.  Obtain a freeze frame of the O2 sensor response to the removal of propane

7. Analyze the O2 sensor response

Remember, a properly functioning catalytic converter will remove O2 from the exhaust gas.  If your catalytic converter is doing its job, there will be a delay of more than 1 second in the downstream O2 sensor’s transition from the rich condition to the very lean condition caused by the propane test.  If both the upstream and downstream O2 sensor respond immediately to the removal of propane by going lean, your catalytic converter is bad and is not removing O2 and hydrocarbons efficiently.

BAT Team Discussions for P0420

  • 2001 Honda Accord
    Error Code P0420 Cat eff. below threshold (Bank 1) 4 cyl. V-tec engine, 01 Accord with 220,000 miles. Should I replace the O2 Sensor in bank one, or is the Cat most likely shot ? And how would I know for sure? Can't pass inspection for emissions....
  • Dodge Intrepid code
    We need mileage and engine and year. Any other codes ? What is repair and tune up history? If it is the cat bad they are under federal warranty for 80k or 8 years. But one of the main causes of cat codes is bad tune up or engine miss or running rich. Also sometimes there is a pcm reflash for the pro...
  • 1999 Honda Accord coding P0420
    Well I've read numerous forums concerning my issue. Pre O2 sensor, cat, post O2 sensor and every combination in between. I have just over 65,000 miles on the car. Called American Honda and they told me my warranty was 8yr or 80K. Of course mine expired in Feb 07. After doing more research I found a ...
  • 01 Toyota Corolla with Cat eff code
    112,000 miles, auto, 1.8l. Has P0420 Cat eff code. Owner reports Cat was replace once under warranty. Graphed post Cat O2 sensor activity and saw it very active, close to same activity as Sensor 1. No other codes or problems found. Fuel trims look good. I'm going to send them to someone for new Cat...
  • 2005 Nissan Altima 2.5
    All, MIL came on on my '05 Altima after 102K miles. Mechanic found error code P0420. I was researching and found the following reference to a TSB" TSB #06004 -- MIL ON WITH DTC P0420: CATALYTIC CONVERTER IS UNDAMAGED. *TT (NHTSA ID #10019113, JANUARY 24 2006) I should note that I've noticed no...

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