Nox

Not sure about Petes or Paccar at all, but there may be two nox sensors that should be replaced, clear the codes, then do a regen.

 

The Paccars that one of the brokers I use have, all have had issues with all the emissions stuff and been in the shop several times for same issues. All of them are for sale

 

Is it the MIL or check engine lamp? Who ever fixed it should have reset the MIL, but if they didn't, it'll stay on for 3-4 days or up to 500 miles.

 

So it's not fixed, that should have been obvious to the mechanic when he couldn't reset the fault, active faults will not reset.

Do you know the exact fault code that they got from it?

 

Be careful repairing it. If it's only a few wires and you've got a little room, you can do a good repair. Use good solder connections that are sealed, this is no place for cheap butt connectors (no place is a good place for cheap butt connectors actually). If there's a lot of damage and you have any questions about the quality of a repair, get a new harness.

These are the only style butt connectors I use:
Butt Connector Multilink
Make sure that you crimp with the correct crimpers that will not pierce the insulation, then heat until the solder melts and they are sealed. A heat gun is best, open flame mini torches are not good for these unless they have the heat deflector attachment for heat shrink tubing.

Is the 1st shop doing anything about the $1200 misdiagnosis? I tell my guys all the time, no matter how simple, go trough the entire diagnostic process. Even if 9 out of 10 times that fault code needs a sensor, that 10th one will bite you. You don't stay in business long if you screw every 10th customer, especially when it's a $500 sensor. It would have taken 10-15 min to follow the troubleshooting and ohm out the harness from the sensor to the ECM.