Help W/2003 Mustang Gt

You need to clear the code. Unhook the battery for a few hours.

What code are you getting? Do you have cats on the car? Are they high flow/aftermarket?

Yes, but what is the specific code? The are hundreds of codes and many of them are closely related, plus many of the codes involve the O2 sensors because it is such an important system.

Still blind, I would suggest tracing some of the wiring to make sure it is in good condition. There is also a heater circuit for each O2 sensor. It would be good to confirm they are getting that signal as well. You can monitor the O2 sensors using a digital multimeter. If the computer is in closed loop (most of the time it is), then you'll see the voltage of the signal wire swing quickly between ~0.8v and ~0.2v. That's normal. Then, at WOT (open loop), it will probably run 0.8-0.9v. This is for the fronts.