Possible PCM fault from Welding Exhaust

Hi, we have got a job at work with an O2 sensor "fault" inherited from a customer who has recently have some hotdog muffs removed from his exh system. The exh flange that was welded is about 1 1/2 inches from the O2 sensors. Vehicle a VT2 GTS 2000.
Initially we thought the O2's had failed (not knowing about the recent welding until later) and I fitted a pair of known good ones that I keep handy and this failed to fix the problem (usual story) so we investigated further.
Unplugging the O2's and measuring the 4 pins with KEY ON , showed 12v on the heater and a good earth on the heater return as expected BUT on the sensor send and return we measured 4.2V on the sensor signal wire and 1.25V on the earth return wire < not correct!
Checking these measurements on a known good car (also in the workshop a VY 2003 R8) showed 0 volts on the sensor send and 0.15V on the earth return > correct. (had to check just in case)
Measuring the wiring at the ECU loom end with the pins removed from the plug showed no back feed from possible wire chafeing through or damage etc ie: no voltage at all. PLugging them back in at the ecu measured the 4.2 and the 1.25V as measured at the O2 sensor plug end. Discussed with customer who mentioned about the welding in passing about 2 weeks ago...which as it happens coincided with the start of the poor running of the engine...
Question: PCM fault from the welding and not unplugging the O2's? Or something else?
Any suggestions or similar experiences welcome.

Regards,
Mike

Hi Mike,

The usual precaution is to disconnect battery, disconnect O2 sensors from vehicle harness, and to place welder ground clamp right next to weld area...

Sounds like that wasn't done and the weld current found "easier" paths to ground...
e.g. if the welder ground clamp made poor contact, or if it was on chassis instead of exhaust, then the ground path may be thru something else like the O2 sensors and the PCM... or thru the alternator and/or battery... or other possible ground paths... or some combination of the preceding.

Typical welding current is 20-30A DC... this will easily punch thru any semiconductor device (like inside the PCM or the alternator).

There may be more damage than just the PCM... this is going to be hard to find all the damage.

put tune into open loop. *problem solved*

yes well interestingly enough it accepts a read and a reflash of the pcm OK and BIDi control into open loop it runs fine. On the scan tool using EFilive the O2's only display 450mV all the time which is the "reference voltage" the PCM puts on the line until the O2 starts switching its own voltage.

Hmmm... the O2 sensor(s) may have been "fused"... if that's all the damage then the customer got lucky.