I have a 2006 XJR which has been presenting the CEL and giving the fault codes P0420 followed a little while later by P0430. Jaguar advised that all four O2 sensors need to be replaced to cure this. They could not really explain why, so I decided to begin by replacing the downstream sensor on bank 1. The most obvious effect that has had is that the long term fuel trim on bank 1 has gone from +7% to -5% and fuel consumption appears to have improved.

First, I'm surprised by the change in LTFT as a Bosch specialist told me that the downstream sensors have no effect on fueling, they only warn if the cats are not working efficiently. Is he wrong?

Second, unfortunately the new O2 sensors have had no effect on the fault codes - P0420 is pending once more.

So what now? Could the Jaguar dealer be right that all four sensors need to be changed to get rid of these codes? It's an expensive gamble if it will not solve the problem. Could it be that it is actually the upstream sensors are past it? Should I try changing the upstream sensor on bank 1 and observing the result? There is no obvious problem with the cats and the car just passed emmissions tests. Is it worth getting a very precise test done on the cats?

Any advice or experience would be welcome.

PS The car has also just had a new MAF sensor.

 

I have never had a problem with o2 sensors but I have some experience with chemical instrumentation (power stations).

It sounds to me that the dealer has got the answer based on their experience although the fact that they can't explain why does not give any confidence.

If it was my problem I would proceed logically.

You have seen an improvement by replacing the downstream on bank 1 therefore replace the downstream on bank 2.

Observe any effects and if you still have the fault codes replace the upstream sensors one by one and look for an improvement.

I think the sensors are only about 60 quid each?
Hope this helps

 

Thanks for the response.

I've discovered that the Bosch agent was wrong about the downstream O2 sensors. According to the workshop manual "The ECM uses this as it's secondary sensor to measure the oxygen content of the exhaust gasses within the exhaust after they have passed through the catalytic converter. As well as providing additional closed-loop fuelling control the ECM uses this information to determine the efficiency of the catalytic converter." This explains why the figures have changed.

The upstream sensors are actually around Ł100, so I'd quite like to be sure there is a problem before changing them. One of them shows 0.1 v on my OBD scanner and the other shows zero v, but they are wideband sensors, so I don't think I'm measuring them correctly. Perhaps if I can work out how to do that things will become clearer.

 

I don't know if this is of any use whatsoever, but I logged the data from the four sensors and made a chart in Excel, which I paste below. The mA data from the upstream sensors was also visible in the OBD software, but it didn't log it and perhaps that is more relevant? This records the outputs from idle, then gradually increasing the rpm to around 3,000 rpm, keeping it there, then letting go of the throttle. The only thing I can observe is that the new sensor (red line) is spikier than the equivalent for bank 2 (purple). Does this indicate nothing or that does it suggest that the new sensor is working better? If there is other data that would be more useful I can try to log it and see if it reveals anything.

 

Most people I've read on-line recommend that the downstream sensors be changed for poor catalyst efficiency codes. I think they are wrong.

For the code to trigger, the voltage of the downstream O2 sensor must track the voltage of the upstream sensor too closely. I think the first course of action would be changing the upstream sensors. Forums for tuning various small cars (Subaru, Honda, and Nissan) recommend installing spark plug non-foulers (spacers) on the downstream sensor on vehicles where the cat has been removed to lessen its response and keep from getting P420 P430 codes.

 

Well, I've now changed the upstream sensor on bank 1 at a cost of Ł100 to see what that does. The immediate effect of that seems to be that I now have long term fuel trim of 0% at idle on bank 1. I'm hoping that's a good sign, but the P0420 code normally took a while to appear, so it's too soon to say if this has worked, but I'll update keep the forum update for the benefit of anyone else with the same problem.

 

Despite my improved fuel trims, it seems this has not solved my problem with P0420 codes. The Jaguar dealer who diagnosed this now suggests that we need to check the wiring from the O2 sensors to the ECM, but says they could not think of any reason why two potentially dodgy sensors on bank 2 should be causing the new sensors on bank 1 to say the cat is not working efficiently.

I've also spoken to a Jaguar specialist who says that in his experience when bad O2 sensors have left the car running too rich for a while that it builds up deposits on the catalytic convertors, which eventually clear up and get rid of the P0420 codes. He's happy to check the car, but advised running it hard for a while first and never using supermarket fuel.

 

Well...well...well... Lucky you ! I had both problems. P420 came first at 98k. I change both o2 sensor " 1 & 2 " it worked .
P430 came second at 108k for cat converter .in California ,the dealer wants 1100$ for new cat and labor $900. I went to custom build ,he wants $800 to rebuild . Go on you tube cost me $22.49 for how to clean the cat converter. You tube told me get one can of " CATA CLEAN " pour in gas tank and drive for 10 minutes when I'm home and tested .I said the cat is clean and ready to smog ,but I have to wait for 2 days for the engine light to turn off. It saved me $2000 bucks from ripoff dealer.
Be happy ,,,

 

What are the best options for O2 sensors? Are there quality aftermarket ones or should we stay with dealer O2 sensors. What's a good price for the upstream and downstream?