99 1500, 360cid, AT, 4x4.

When the exhaust gets hot (usually on long trips) it idles very rough and the check engine light comes on. I borrowed an OBDII scanner from the local Chief Auto Parts and it returned error code P0171:

"A lean air/fuel mixture has been indicated for fuel system 1/1 by an abnormally lean correction factor"

I just tuned the truck up (cap, rotor, plugs, oil & air filters, etc.), and just had the cat replaced. The timing appears correct? (around 5 degrees advanced, but I don't have a manual so I'm guessing based on other V-8's I've owned in the past). The truck runs fine on cooler days; I just drove it 120 miles this morning- no problems.

Does anyone know exactly what this code means? Is it the front O2 sensor reading wrong, and telling the computer to compensate in the fuel mix?

It sounds like the heater circuit on the 1/1 O2 is bad. This may happen intermittenly, causing the computer to think the engine is running lean. I would replace the O2.

does the vehicle have an exhaust leak!!

No exhaust leak...just had the cat replaced, it was clogging.