The dreaded O2 Sensor
 

First post - I've read these forums for help for a long time, now I've got a problem I can't solve.

A few months back my Cherokee 96 (SE, 2 door, 2.5L) had the throwout bearing blow, so I took it to a shop. Had a full clutch kit installed, somehow they broke my downstream O2 sensor when they were dropping the transmission.

Since then, I've had the MIL on (literally since I drove out of the place), and I'm getting a P0138 code thrown (high voltage in downstream sensor - it reads around 4v from ECU). Checked the voltage on the heating circuit (from the connector at the sensor, and the connector where the wiring joins the harness in the hood), and I'm getting around 1.8v rather than the expected 12-14v. Wiring along underside of car seems fine based on these results and visual inspection.

Now I assume this is a short or busted relay (since it's occurred after they broke the old sensor removing it). My question is - where to I test next? I don't have an OBDII scanner, but I have a multimeter so I can check continuity/voltage. Since the wiring joins the rest of the harness in the hood, I'm not sure where to follow it next or where I'd find the relay it's getting power from. I've scoured the wiring diagrams I could find, however I'm not sure if they're relevant to the vehicle I'm working on.

Just a practical suggestion: I assume you were paying the shop in which case they broke it, they fix it.

Not really sure why you waited a few months. I wouldn't have waited a few minutes before i went back in there and insisted that they either repair it while i watched, or payed to have it done elsewhere.

Did they tell you they broke it? Or how did you determine this?