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There are two separate grounds as far as the ECU is concerned. There's a low-current ground shared by many of the analog sensors in the car and a high-current chassis ground. If you install a high-current device, like a WBO2 kit for example, powered off sensor ground or you short an O2 sensor somehow, you can blow this ground track inside the ECU. If the track is burned, you need to fix whatever caused it to burn or else after repairing the track, you'll just blow it again or, worse, blow something else in the ECU if your repaired track is stronger than the original.
A blown sensor ground trace will be evident in datalogs. It will result in -40F intake and coolant temp readings, 100% TPS, an unusually high Baro reading (like 37 inHg) and O2 voltages greater than 1V. The car will also most likely not even start.
Sensor ground is pin 92 on a 2G DSM ECU and pins 17 and 24 on a 1G DSM ECU.
You can confirm a blown sensor ground track by opening the ECU and examining the tracks near the sensor ground pins. The picture below shows the location and proper repair of a blown sensor ground trace on a 2G DSM ECU. The 2G ECUs almost always blow the ground trace right at this point. The 1G ECU, however, can blown the track at a couple different spots. You'll need to search around a bit more if you've got a 1G.
If you repair this yourself, be sure to scrape the green solder mask and white silk screen from the board to reveal bare copper before soldering.
For a 1G ECU, the symptoms are the same, but the burned track can be on the top of the board connecting to ECU pin 24 or on the bottom of the board in the are beneath IC101.