I had a diagnostic test on my S Type and was told it had quite a few things that had to be repaired. It all started with a loss of power when driving up hill. I'm told I need two new cats, four Lambdas new plugs and ignition coils, all a result of engine backfire. I bought all the parts, only to be told that it could just be air loss from one of the many pipes attached to the engine. If all these parts need to be replaced, would the engine still run, and would the tick over be erratic? I ask because it starts first time, idles well and is not frowning out lots of fumes from the exhaust. If you have any ideas, please let me know. Bojag.sjr.

Get an indy to check it out, mich cheaper than a main dealer and he will want to keap you as a regular.

The plug are a common issue as they are hard/expensive to replace and cause miss fires and poor pulling. Replacing all the coils is a clear sign of bad diagonise. Normal a plug change and test each coil pack.

Air leaks can be detected quite easly with basic diagnostic equipment by a compitent person and would not be confused with a plug/coil pack issue.

I had a 2.5 s-type and I would get the plug changed and the coil packs tested first, mostly to solve your issues