Had 2 new catalytic converters installed on my 2004 jaguar xj8, new spark plugs. Then the car now backfires. What could be wrong?

 

Many things could be wrong. You need to check for codes via a scan tool (or have them read at a local parts store, that will usually do it for free). Using the OBDII codes provided on this forum you should be able to identify the problem. Even if you are not able to repair the problem yourself, if you post the codes more help can be provided via the forum. Good luck. I would check for bad connections near the convertors or spark plugs. The powertrain (P) codes are needed. (Also, who did the work and how many miles are on your 04 and what else has been done?)

 

Car has 150k miles. I had been using the Jaguar dealer but was not happy with my last two experiences with them. Work was done by a "reputable" garage which specializes in European cars. The 4 oxygen sensors were changed and new spark plugs too. I had replaced the ignition coils about 4 months ago and also had new spark plugs installed then.

There are no error codes showing when I use my scan tool.

 

I would hazard a guess that since the only "things" touched when the cats were replaced were the O2 sensors and plugs . So I would look at those sensors and check wiring and also the make
of the O2 sensors and make sure they are appropriate for your car.

Why did you have new plugs installed 4 months after changing them?

 

I had the Jaguar Dealer do a diagnosis and just followed what they had indicated they were going to do. The sensors are supposed to fit/work with the 2004 Jaguar XJ8. I will check the connections and the plugs.

thanks

 

don't skimp on cheap sensors . use OEM . for coils as well . LESSONS I HAVE LEARNED !
check your fuel trims and MAF rate also . can be done on a $15 dongle and a smart phone .