Hi, I have a 98 explorer with v8 and auto. About 169K on it now. Bought it a few months ago, and person told me it had slight hesitation or miss going uphill, or when there was a load. He thought it would just need a tune up. Since then, I have not replaced the plugs or wires yet, but once in a while the check engine light would come on for a bit but then off again. Since the last few weeks now and it has been much colder out, it has gotten worse. The check engine light stays on all the time now, and the miss has gotten much worse. It does it at idle and especially around 40 to 50 mph after it just shifts into overdrive. It just feels like a bad miss but doesn't do it all the time. It would run rough driving to a gas station, shut it off and run inside, start it up to leave and it runs smooth as can be.

Took it autozone for a code check today, and there was 2 codes.

P0340

P1309

He talked me into buying a camshaft sensor for $23.99. Can a faulty sensor cause this. Anything else cause this. He didn't think a tune up would help. I don't know any of the history of this vehicle and I bought it with around 165K miles. I havn't looked around yet to find the camshaft sensor. Can anyone tell me where the sensor is? It runs good when you first start up but it gets worse as the engine warms up.

Also, anyone have any info on replacing a heater core for this vehicle. I would like to try, but don't know. I changed one out on a 92 S-10 so I think I can do it, if someone has some steps or info on doing it. I don't have a book for this explorer and the wife wants to swap and get a Durango (for the kids) so I am not going to keep it for that long, but I do want it running right and fixed.

Thanks