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Diagnose P0420 Catalytic Catalyst Code

by Larry Carley copyright

Your Check Engine light is on and you find a P0420 low catalyst efficiency code for your Catalytic Converter. Does that really mean your converter has reached the end of the road and needs to be replaced? A new original equipment converter can cost up to $2000 or more, while an aftermarket converter may set you back $500 or more. It's an expensive fix so you want to make sure your vehicle really needs a new converter before you replace it.

A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) catalyst monitor detects a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in the exhaust manifold reacts to the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reacts to the exhaust gases as they exit the converter.

Catalyst Monitor Efficiency Self-Test

The OBD II catalyst monitor runs a self test when certain driving conditions have been met. When this test occurs will vary depending on the year, make and model of your vehicle. The engine and converter have to be at operating temperature, and the engine may be idling or running under light load at low speed. During the catalyst self-test, the engine computer makes the air/fuel mixture rich temporarily to deplete any stored oxygen in the converter. Then the computer makes the air/fuel mixture temporarily lean to determine how long it takes for the converter to react and for the downstream O2 sensor to change its switching activity. If the converter takes too long to come back to life, it means the catalyst is not working efficiently enough to reduce emissions. The OBD II system will fail the converter, set a P0420 trouble code and turn on the Check Engine light.

Catalytic Converter Operation

The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.

The OBD II system can't actually measure the concentration of HC or CO in the exhaust, so it compares the upstream and downstream O2 sensor readings during the catalyst monitor self-test to determine how well the catalyst is doing its job. If efficiency has dropped below the cut point the vehicle manufacturer has established for the application, the converter fails the test and a P0420 code is set.

The upstream O2 sensor will undergo a lot of switching activity because the engine computer is constantly adjusting the fuel mixture between rich (more fuel) and lean (less fuel). When the engine is first started, the catalyst is cold and doesn't do anything. During this time, the switching activity of the upstream and downstream O2 sensors are essentially the same because nothing is happening inside the converter.

When the converter reaches about 600 degrees F, it is hot enough to start reacting with the gases in the exhaust. This is called the catalyst's "light off" temperature. The converter now starts to clean the exhaust and remove the pollutants. This causes a sudden drop in the switching activity of the downstream O2 sensor, and the downstream O2 sensor's output voltage levels off to an average reading of around 0.45 volts.

If the catalyst monitor runs its self-test and finds the converter is functioning within acceptable limits, the vehicle is in emissions compliance and no codes are set. But if the catalyst monitor finds efficiency has dropped off and the converter is slow to respond, it may set a P0420 code and turn on the Check Engine light.

Does a P0420 Code Mean You Need a New Converter?

Yes, it usually means you need to replace the converter - but not always. You may have a false P0420 if your vehicle's catalyst monitor is overly sensitive. Some vehicle manufacturers have issued Technical Service Bulletins (TSBs) that involve reflashing the vehicle's engine computer so the catalyst monitor won't be quite and sensitive and slower to fail an aging converter.

Other conditions may sometimes lead to a false P0420 code, such as an exhaust leak (which may fool the O2 sensors), fuel pressure problems (too high or too low), or a problem with one or more O2 sensors. But if everything else is working properly and a P0420 code is setting, the converter is not meeting emission requirements and it must be replaced to pass an emissions test.

Most states now use a quick OBD II plug-in test to check emissions compliance on 1996 and newer vehicles. The plug-in test is faster, easier and cheaper to do than a loaded mode tailpipe emissions test on a road simulator or dynamometer. The rules in most states say to pass the OBD II plug-in test, the vehicle (1) must have a fully-functional OBD II system (Check Engine lamp works and the diagnostic connector communicated with the engine computer), that the Check Engine light must be off (not commanded on), and that there are no current codes in the computer's memory.

Consequently, if the Check Engine light is on and you have a P0420 code (or any other code), you will FAIL the test -- unless the state allows you to take an alternate tailpipe emissions test to see if your vehicle is actually polluting. Some states allow this but others do not.

If your state allows an alternate tailpipe test -- and your car passes -- don't worry about the converter code. Your OBD II system may be over-reacting to a perceived emissions problem that does not yet exist.

Causes of Catalytic Converter Failures

Under normal use, the original equipment converter on your vehicle should last upwards of 100,000 to 150,000 miles. But any number of things can make it fail prematurely. The most common cause is contamination of the catalyst because the engine is burning oil or leaking coolant internally (leaky head gasket or a crack in a combustion chamber or cylinder). Converters can also be damaged if they overheat due to ignition misfiring that allows unburned fuel to pass through into the exhaust (check for a fouled spark plug or bad plug wire). The same thing can happen if the engine has a bad exhaust valve that leaks compression into the exhaust (check compression).

Passing an Emissions Test

If you live in a state that does not allow an alternative tailpipe test if your vehicle fails an OBD II plug-in test, you have to get the fault fixed to pass the test. That means replacing the converter even if it is marginal or still functional but not working well enough to make the cut point.

You can't just erase the P0420 code with a scan tool prior to taking the emissions test. That will turn the Check Engine light off, but your vehicle won't be allowed to take an OBD II plug-in test until it has been driven long enough for all of the OBD II monitors (including the catalyst monitor) to have run and completed with no faults found. If the OBD Catalyst Monitor is Not Ready, you can't take the test. And if it runs and finds the same problem, it will reset the same P0420 code and turn the Check Engine light back on.

So what's the fix? Nine times out of ten, a P0420 code means you have to replace your converter(s).

Aftermarket versus Original Equipment Converters

Aftermarket converters are less expensive than original equipment converters (1/2 to 1/3 less!). But aftermarket converters usually contain less catalyst and have a much shorter catalyst bed inside the converter. Consequently, they are only guaranteed for 2 years or 24,000 miles - and in some cases they may not even function well enough to prevent the P0420 code from being reset! Or, they may not last until the next required emissions test.

If you do opt for an aftermarket converter, get a "Direct Fit" converter that bolts right in the same as the original. "Universal" converters fit a wider range of makes and models and may be less expensive than a direct fit converter, but they often require adapters and/or cutting or modifying pipes when they are installed.

Don't waste your time installing a used converter because there's no way to know what condition it might be in or how many miles are on it. Besides, used converters are now illegal to install on OBD II vehicles in some states (like California).

Removing the converter and replacing it with a piece of straight pipe or a "test pipe" is also illegal, and the OBD II system will detect the missing converter anyway.

Worst case scenario: you replace the converter with a new aftermarket converter, drive your car awhile and the Check Engine light comes back on. The P0420 code has returned. Now what? Clear the code, drive the vehicle until the OBD catalyst monitor runs again, and if it doesn't come on hurry up and get your vehicle inspected before the code returns. Or, redo your diagnosis to see if you missed something like an exhaust leak, fuel pressure problem or fault O2 sensor that is fooling the catalyst monitor. Or, remove the aftermarket converter and replace it with a new original equipment converter.

Oxygen Sensors

For more information about oxygen sensors, see Understanding Oxygen Sensors

If the downstream O2 sensor is bad (heater circuit not working, loose or corroded wiring connector, contaminated sensor element, etc.), the OBD II system should detect the fault and set an oxygen sensor code. This should prevent the catalyst monitor from running since it it needs a good signal from the upstream and downstream O2 sensors. But sometimes a faulty O2 sensor is not bad enough to set an O2 sensor code but is off just enough to affect the accuracy of the catalyst monitor.

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Catalytic Converters: How They Work & How To Diagnose

Catalytic Converter Thefts Soaring To New Highs!

How to Check Exhaust Backpressure

Understanding OBD II Driveability & Emissions Problems

Fixing Emission Failures

OBD Catalyst Monitor Not Ready

Trouble Code Diagnostic Tips

Understanding Oxygen (O2) Sensors

Wide Ratio Air Fuel (WRAF) Sensors

Sensing Emission Problems (O2 Sensors)

Emissions testing update

OBD II Emissions Testing

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