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Land Rover Discovery DTC P0420

Q. Hi, I have a 1997 Land Rover Discovery, 4.0 liter V-8 and the check engine light came on. I hooked the scanner and came up with code P0420 - catalyst system efficiency. Where do I go from here? I am a "do it yourself" type. I have done a lot of work to my other toys (1978 Jeep CJ5,1987 Mazda RX-7, 1966 Mustang, 1990 Chevy 2500 truck).

Land Rover Discovery DTC P0420

The Discovery belongs to my wife. Even though I have a scanner to tell me the code, I am not sure what to check (plugs, injectors, converter). I also have a computer codes manual that list all kind of code charts, but it does not tell you what to look for after it list the code meaning. Can you help me? Do you have any suggestions for a manual or book I can buy to learn more about this?


A. DTC P0420 - P0420 - Catalyst Efficiency Low Fault Bank A. What that means is this, there are four Heated Oxygen Sensors (H02S) in the exhaust system. One in front of both catalytic converters and one after. The PCM takes the O2 reading from the front sensors and compares it with the rear sensors. If the difference falls within pre-programmed parameters, the catalytic converters are deemed good.

If the difference between the front and rear O2 sensors falls outside of those parameters, then the converters are deemed bad and the MIL comes on. For instance if there is no difference in the front and rear O2 sensors then the catalytic converter is not doing any thing. Or if there is a very large difference then it's possible the catalytic converter has a hole in it.

Now in this particular case, the PCM had come to the conclusion based on information from the front and rear O2 sensors that the catalytic converter in bank A is bad. So it will probably need to be replaced. Bank A is the side that the number cylinder is on.

Additional Information provided courtesy of ALLDATA

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