I have a 97 XJ6 Base. The car runs beautifully. Twice now I have had the check engine light come on and it shows the P0420 code. I've driven several trips on the interstate and highway with no issues. Both times the light has come on I was in my driveway with the car idling for approximately 4-5 minutes. Have any of you had this code pop up and what was the issue/solution. My Jag only has 57,700 miles on it and prior to my ownership it sat for long periods of time. I don't know if this may be contributing to the engine code or not. I would appreciate any input or suggestions.
Thanks!

 

Hi Dave,

P0420 is a very common code and the definition is "Catalyst Efficiency Below Threshold, Bank 1." On your Jaguar with the straight six engine, Bank 1 represents the forward three cylinders of the engine, cylinders 1, 2 and 3, counting from the front of the engine backward. Here's the listing from the Diagnostic Trouble Codes Summaries with some possible causes. Another possible cause inexplicably missing from the list is failure of the catalytic converter:




Cheers,

Don

 

Thanks Don. That's a good concise list to check on. Since the P0420 code a fairly common as you stated is there one of these possible causes that seems to be more common than the others? If so, I will probably start there. Thanks!

 

Dave,

All of the possible causes listed can trigger P0420. However, if your code were caused by an intake air leak, there's a good chance you would also have one or more codes for lean running (P0171, P0174, etc.). Likewise if the MAFS was dirty or out of range. But the OBDII system in the X300 is very rudimentary and can't be expected to always behave rationally (1996 was the first year OBDII was mandatory), so you really need to rule everything out.

A quick way to test the catalytic converter is to use a non-contact infrared thermometer to measure the temperature at the inlet to the cat with the engine running at full operating temperature, then measure the temperature at the output of the cat. Avoid heat shields and try to measure right on the cat body where the exhaust pipes connect. You're looking for the output temperature to be significantly hotter than the input, usually at least 100 degrees F hotter. If the temperatures are similar, or the output temp is colder than the input, then the catalytic converter is not working.

To observe the oxygen sensor signals you will need a scan tool capable of reading Live Data on your car.

One sign of an exhaust leak is dark sooty staining somewhere around the exhaust manifold. Leaking gaskets and cracked manifolds are not uncommon.

Cheers,

Don

 

If there are no exhaust leaks present, DTCs P0420 or P0430 can usually be corrected successfully by replacing the catalyst for the respective bank.

 

This is excellent information and I thank you for the help. I do have a scan tool that show me live data. What are the normal range readings of the o2 sensors? I can easily check these first then move on to the catalytic converters if necessary. The engine itself it clean with no indications of a leak around the exhaust manifold. I do have a can of Mass Air Flow sensor cleaner so I can go ahead and clean it and the housing as well.

 

O2 Sensor voltage on the X300 should switch between 4.7v and close to 0v. If your scan tool is a cheap and cheerful ELM327 with a phone app, it's likely that it will only show up to 1v max. But that shouldn't matter.

What you are really looking for is to see the switching between high and low. This should be at least every second. If it's slower or erratic then there's a problem somewhere.

You can test if a sensor is working by inducing rich and lean conditions and observing the sensor voltage reaction.

1. Pull off a vac hose to give an air leak. This is a lean condition and you should see low voltage on the O2 sensor.
2. Put the vac hose back and let the idle stabilise. Now spray some easy start fluid in to the intake. This will give a rich condition and you should see high voltage on the sensor.

If you have a good bank (4,5,6) and bad bank (1,2,3) then compare the readings. Both banks should react the same.

 

Thank you for the detailed information. This is a huge help. All of you on this forum are going to make me a Jaguar Forums Certified Mechanic. Tomorrow I will dig into this and see what I come up with.

Thanks again,
Dave

 

Heres some screen shots from my 06 STR. Different levels for sure but I could see one bank was different so it helps narrow it down.

these are after I fixed it