I have a 1998 Discovery I with 95,000 miles. It started to misfire, sputter and the check engine light came on so I took it into the shop and I was told that I was in need of a major tune-up, the Cam Shaft Sensor needed to be replaced and also the Catalytic Converter had failed and needed to be replaced. I am ok with that, but my question is if I give it a major tune-up and replace the sensor do I need to change the Cat right away? Or will it still misfire with the old Cat in place? The Cat is not a cheap part in California. Any advise would be great. Thanks!

 

A cat will not be the cause of a misfire in any vehicle. A clogged cat will create excess back pressure, but that should show as a loss of power not a missfire. It is rare to get that badly a clogged cat on such a new vehicle. Cam position sensors can fail and it can cause a stumble and even a stall. You often see this when coasting to a stop as the engine cannot properly read how fast it is going and wether to slow down or speed up. It feels like a lope or often described as a stumble that settles out when truck comes to a complete stop and idles. I have had them fail on my other vehicles and it feels distinctly different than say a bad plug. I would do tune up only and see how it runs- suspect a bad plug first. If you are handy at all, pull the plugs and replace them. Inspect ignition system if you are capable. I would not let a stealership touch any vehicle of mine unless it was a major wiring issue. If you are not mechanically inclined, have them change the plugs if that is included in the tune up. go out and listen to it and hear for yourself if it is not resolved before proceding. Is your truck blowing smoke? if not, cat is prob fine and they are trying to take you on a ride that ends in your wallet being empty.

 

Think of it this way: the proper way to do anything is to fix one thing at a time starting with the one most likely ( plugs/ ignition syst in your case). if you change mult things, how do you know they all needed fixing and you did not get ripped off? I say again the cat being the cause is rubish. I took one out of my powerstroke ( diesels have much more soot that can clog up cat) and it was pristine at 160k. you would have to be blowing a larger cloud than my diesel and that is just not possible. A cat is a series of baffles that are superheated by exhaust gasses to burn of any additional exhaust. It a static device in terms of moving parts - nothing to break, it can only clog and that is not common esp at 95k on a gassser.

 

make sure the valves arent sticking plug wires hopefully this big trouble on these cars

 

Would agree with earlier post, a faulty CAT may well fail emission test but has absolutely nothing to do with misfire. I would give the shop you went to a very wide berth. If check engine light on then a sensor is out of range this could be something very simple start by getting the fault codes read then this may point to the problem, then try the cheap things like plugs and leads. I think most of all find a decent repair shop a few hours phoning/driving around may be worth while.