That's the heater control circuit and I've had this on and off for a while, replaced the sensor and came back, after digging around that's usually a wiring problem, I've ordered a delco pigtail to splice on since those wires look toasty from the catalytic converter. Yet to put it on though. It comes on for a day then goes back out then comes back. Since they had your exhaust off the probably pulled on that when they disconnected it.