What is causing my Service Engine Soon light to stay on?

Some systems require a 'reset' for the ecu. Most of the time disconnecting the battery for more than 10 minutes will do the trick. Some of the mustang forums suggest: 1) disconnect the battery for 15 minutes, 2) reconnect the battery and start the vehicle, 3) let idle until warm, 4) turn A/C on (if equipped) and idle for 5 minutes, 5) drive the vehicle for 10 miles. If that doesn't work have the dealer reset it. Depending on the year there are different reset methods. If it still stays on run the codes again.

I have duel catalytic converters. As for the reset, the battery has been disconnected a couple of times. When I go to the dealer they would want to change the hot air oxygen sensor, (That's what shows up on the computer), but that has already been changed.

If resetting the ECU has no effect, then Tom may be right. But if the O2 sensors have been replaced and it still codes then look for the obvious like a frayed/shorted wire or a bad connector. You say you had the headers replaced. Were they replaced with stock or aftermarket parts? Has anyone done a test for a shorted or broken wires?

The headers are after market, purchased a couple of months ago, but I've been having the service light problem since I had the car.

I really don't know what to tell you to do. Something is definately amiss. Usually if you fix the code and the problem persisits it is something in the wiring either broken (open circuit) or shorting to ground. I have heard of problems as strange as being in the dashboard itself. Either way it is going to take time and patience to narrow down what circuit is causing the problem. There is a cheap fix, and I shutter to even suggest it. Just remove the bulb.

Yeah I suggested that but my mechanic advised against it. However to me for the light to be always on is the same as it never being able to come on. Either way it is not telling me anything.

I HAVE a 1997 gt mustang and my engine light came on had the mechanic check the code and it was just the gas cap so I had to buy a new one and the light went off

Will it pass inspection in new jersey if light stays on?

My engine light has been on for 3 yrs and Ford is telling me its a thermostat issue sending the code, change it and still comes on, also as headers but it works fine but really strong rich smelling exhaust, any ideas?

I've had my 04 GT for almost 6 months consistently now I completely replaced the block from a 03 crown Vic and the transmission came from what seems like a truck (torque converter due to it being too large for the mustang, and just today my check engine came on. I noticed it maybe a couple months ago but my ac leaks right onto my passenger side exhaust pipe and last night I was noticing a whistling noise coming from underneath my car. My question is could my ac bust a hole into my exhaust causing the check light to be thrown up?