I have a 2001 expedition XLT 4.6 liter 2-wheel drive with 134,000 miles, and I have been having problems with the Service Engine light turning on and off randomly, but over the last month it is stayed on continually. It's not the gas cap, I wish it were that easy. Here is a little backstory:

About 6 months ago the engine started running roughly, and the Service Engine light came on for the first time, but I couldn't do anything to check it out for about 2 weeks. When I finally took it in for a reading, I found out one of the spark plugs was dead. I bought all new plugs and replaced them, and I found out why plug 7 was dead, there was water in the chamber where the plug sets. I dried it out, then replaced the plug. Everything seemed fine after that, but about a week later, the Service Engine Light came on again. I had it looked at and the guy at the parts shop said it looked like my catalytic converter was having problems.

Well, since then, the light has been going off and on, sometimes staying off for a week. I took it in, again, a few days ago, and I got the general readings from the reader at Auto Zone which are these (it could be only one of these, or more than one):

TROUBLESHOOTING PO430
The PCM has determined that the catalyst system efficiency for bank 2 is below threshold for the current engine operating conditions.
PROBABLE CAUSES
1. Catalytic Converter defective
2. Engine misfire or running condition
3. Large vacuum leak
4. Engine oil leakage into exhaust-valve guide seals, piston rings.

TROUBLESHOOTING PO401
The PCM has determined that there is insufficient flow in the exhaust gas recirculation system.
PROBABLE CAUSES
1. Blocked passage in EGR system
2. Failed EGR valve
3. EGR solenoid faulty
4. Failed EGR pressure sensor where applicable

I am not a mechanic, but I want to try to fix this problem myself, if possible. What should my first step be in troubleshooting, starting from the least expensive up to "take it to a mechanic." Sometimes my truck's engine runs smoothly, and other times it's rough.

Thanks

 

Welcome aboard.

You've got several of the more common fault codes, both very well discussed.

You can likely dig up relevant info from old posts by clicking on the SEARCH button, then enter the fault code as your keyword.

You need to fix that intermittent misfire. You've probably loaded one of the cats (driver's side) with unburned fuel and it's now "cooked".

Steve

 

Thanks. I did try a search for "Service Engine" but it returned nothing. I'll try the codes.