is oxygen sensor heater. I put on a header and had to extend the wires because the bung was at the bottom of the downpipe right before the cat. Any ideas why this code would go off? Should I re-check and solder the connections instead of butt connectors?

 

Definitely solder and ensure you're using good materials (wire and solder). Physically extending the wires can actually cause a lot of issues with the O2 sensors in Mustangs b/c of the way the reference oxygen through the wiring. I would assume the same holds true for these cars.

Can you not purchase pre-manufatured wire extensions or different O2 sensors with longer pigtails? The longer sensors are often used on other models of the same make.