I also have code PO131 High Voltage and suspect the alternator.
It's an OBD1 computer.
I suspected the fuel filter so I changed it.........twice.
The fuel pump failed a few months back so that is new also.
MasTec 1970
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Part of the problem here is a lack of communication! This is because of who ever gave you the DTC information was misinformed, P0131= B1S1 (Bank 1 Sensor 1) O2 "Circuit" Low Input, and P0171= System lean (Bank # 1) Where did you think that the alternator had something to do with it? Just replace the B1S1 Oxygen sensor and both of these codes will be gone!
NOTE: You have an OBDII system on this vehicle because a lot of 1995 GM's started using OBDII before it was mandated in 1996! All P**** are OBDII style DTC's, GM OBDI systems have 2 digit codes and 2 or 3 digit codes on Ford!
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the fuel trim lean code is probably due to a plugged fuel filter. and the high voltage is due to the regulator in the alternator failing. plugged fuel filters are the prime suspect in lean codes.