This is the code (P0171) that came up when I took it to auto zone. The truck is running rich and will hardly idle in drive. The rpms drop down to about 300. It seems to be worse once the truck is warmed up. Any suggestions as to what it might be.I took the throttle body off and cleaned it but it didnt help.

P0171 Should be a Fuel system too lean Bank 1 code. IIRC by the OBD II years there should be bank 1 and bank 2 oxygen sensors. Some early OBD II models still ran with one upstream oxygen sensor though. Check for left and right oxygen sensors. If there is just one, replace it. If there are two, bank one is the drivers's side. It sounds like the oxygen sensor is stuck at full lean, and the computer is compensating for it. I would replace the said oxy sensor.

Hope it helps,
Bill

I just pulled out the #1 and #2 spark plug and they are as black as coal.

There appears to be one o2 sensor in the cat. and one down stream from the cat. Is the one in the cat the one that needs replaced?