Before the 24 Hours of Lemons race, I got the car running and it would not run right. I pulled up Torque, and I was not getting engine temperature, boost pressure/vacuum, as well as a bunch of other issues. The car would not really accelerate either. Eventually I got some codes, P0238, P0236, P0118, P0103, P0134, P0452, P0113, as well as a bunch of pending codes. I think the P0452 has to do with the missing evap solenoid. These indicate problems with the coolant sensor, Looking at the wiring diagram for the car, all of these sensors shared a purple and brown wire. Measured the voltage on these wires, got about 8v on purple and 5v on ground (I believe, not that important actually). The one forum post I could find, (https://www.matthewsvolvosite.com/forums/viewtopic.php?f=11&t=69017)[Forum post] was for a Volvo S80, but they just replaced the ECU. I didn't want to do this, and did not have everything to clone one, so I dug further.

The power for these sensors all came from ECU, so I figured the best thing to do would be to rip it apart and see if I can replace anything on it. The ECU is easy to get to on my car because its all stripped out. You need to push the pink tabs under the ECU forward. There is one on each side, and you push one back about 3mm, then the other. I used a T25. There is a fancy tool, but $$. There are 4 screws on the back, T25. Looking at the board, maybe this corrosion is a problem... Oh. Oh no...

Closeup of ECU showing corrosion on QFP chips
Corrosion on ECU chips
Burned trace on the bottom of the ECU
Oh. Oh no, burned trace.

On the back was a burned trace, looked like it completely burned through. If you are at this point, you NEED to figure out why this burned up, or it will burn again. I believe that a purple and a brown wire touched on mine. On a previous engine out, I pulled out an AC sensor, and the wires were floating in the breeze. Check all the sensors on the schematic, IAT, Coolant temp, Boost pressure, front AC pressure (?), MAF. I cleaned the trace off, pulled as much copper out that was burned up (Be careful), and soldered two new wires. There is a pad on one end, a via, and a capacitor on the other end.

Wire going from the edge of a resistor to free space
First wire soldered

I used a bunch of flux on all pads. Also to make soldering easier, I used some leaded solder on the 1002 resistor, as well as additional flux. You'll need a fine soldering iron. In the above picture, you can see where I soldered to on that trace. The top bit of that solder ball between the 4 unpopulated pads is the via. To scrape the soldermask off, I used a "eurotool" fiberglass scratch pen. If you're careful, it will scratch off the soldermask, but not the copper. Flux and leaded solder is your friend here. I used fine wire, wire wrapping wire, but anything will work.

After soldering both wires, I used hot snot to secure the wire and keep stuff waterproof. (Ugly, but its a 500$ car. Who cares)

Hot glue covering the bodge wires
Hot glued wires

Cleaned the corrosion, buttoned up the ECU, and all the problems went away. Yay me! The following pictures are all the chips and ECU marking for comparison.