www.jec.org.uk

Hi everyone , my 1997 Jaguar XJ x300 has failed MOT on Emissions , the co reading is 10 and apparently should be 0.03. I have replaced the N/S centre exhaust box as this was blowing and still the reading is 10. There was a lot of water in the exhaust when I replaced the box don't know if this is any significance . The engine runs sweet , the spark plugs have been changed and the fuel system has had some expensive cleaner in it. I was going to try the lambda sensor next but I am only an amateur and was wondering if anyone has the same problems and or know of a solution. Thanks in advance.

I would think Lambda sensor. If you have an OBD reader or know someone with a reader, the fuel readings won't be even between the Two banks of cylinders. The 'Bank-1' reads 1,2,3 cylinders. 'Bank-2' reads 4,5,6 cylinders. Both should read about 93-97% efficient, if there is a fail the failed bank will read about 24-30% efficient, that's the bank that's failed. When you look down towards the exhausts from the drivers-side of the engine compartment, the furthest Lambda is bank 1 , cylinders 1,2,3. The connectors for the Lambda's are at the passenger-side of the engine compartment attached at the rear of the cylinder-head, unless (as I have done with many X300's) they have been moved to the drivers-side well proud of the engine and obvious connectors with four wires each. You can't just swap the wires/connectors to check if one has failed, this confuses the ECU. You would need to swap the position in the exhaust downpipe of the sensor to check if the lambda had failed. Of course if this was the case, then the reverse readings on the laptop between the banks 1 and 2 is the case, the bad reading is the 'failed' sensor. I have all the diagrams for this work, and the fuel efficiencies, along with pics of the work I did to adjust the position of the connectors, to bring them to a more accessible place in the engine compartment. I'm away on a work related course for a week from tomorrow, if you still have difficulties say here and I'll try to help, but from a distance is trying. It's an easy task to change the lambda's though albeit expensive for the sensors. One last thing, check your fuel-consumption, that failing sensor 'bank' will be dumping extra fuel into the cylinders, so you'll be down, probably to about 17-18 mpg to the gallon.
Hth
Peter

I'd also guess at lambda sensor but certainly wouldn't change it (or anything else) until I knew it was malfunctioning.

Agree with the suggestion to check it with a simple OBDII reader (I use an App on my phone). But instead of checking fuel readings, just look at the voltage readings to see that it's cycling as per the other sensor.

If you don't have an OBDII reader, you can even just check the voltage reading by putting a multimeter on the plug where the sensor cabling is joined at the top back of the engine (on an XJS, guess similar on an x300).

Good luck

Paul

40 and 300 are renowned for failing mot on emissions, before ploughing any money into the car, use my trick, book your mot, 30 minutes before take it for a good run, keep the revs up, I use 2/3rd gear only around the lanes here, get the car properly hot, it's called the "Italian Tune Up" drove it like you stole it, this gets the lambdas working correctly, clears the cats, then when you arrive for your test, get the emissions done FIRST, then they usually pass first time