This car had repeat ignition coil misfires on multiple previous occasions. This causes engine knock and the car to go into limp mode to get you home. I have seen many forums on this problem on the Jaguar designed 3.9 Liter V8. This problem, although you fixed the ignition coils (under warranty) then manifests itself as a problem down the road into two separated but related problems.

First of all, as the unburnt fuel (because the spark plug failed to ignite) causes unburnt gasoline to go down the exhaust pipe and the super-hot temperatures cause it to melt the honeycomb of the catalytic converter. You will get code P402 or P402, a Catalytic converter below peak efficiency code. You can temporarily pass emissions by using a new oxygen sensor post cat (not pre-cat) on the engine bank involved. This is a $70 repair, not an $1,800 dealer repair. But the problem will not fully go away. As the catalytic converter gets blocked, further and further, it causes excessive back pressure and the engine to overheat, because the heat of the exhaust is trapped and can't go down the tail pipe. This can cause a serious overheating problem, leading to a blown radiator overflow bottle, since it is plastic and it ruptures from excessive back pressure. (happened to me twice, and I replaced thermostat and thermostat housing and bled the radiator as well). In fact, it's the back pressure of a partially clogged exhaust responsible for this all, and thus, you can wind up with a blown head gasket if you continue to repeatedly overheat. Eventually, the repair from a dealer will cost you $1,800 per side, .

I found that Walker exhaust cats are cheaper but quality non-Ford parts, and with labor will only cost $1,000. But this leads you to very poor engine performance, loss of gas mileage, and chasing your tail for overheating problems, which are in reality , simply excessive clogged catalytic converter back-pressure problems. All these repairs wind up costing me $2,300 combined. So it's a bad coils---->bad cat--->clogged exhaust---->overheating engine.

Problems: repeated issues with coils. Repeated issues with overheating. Bad Catalytic

Eventually, even if you drive with check engine light on, and diagnostic code is P402 or P403, you'll have to bite the bullet and get almost a complete new exhaust, with new cats. This is on a $42,000 to $45,000 car that was new, at only 8 years of age.