Hi all, hoping for some input before I throw more parts at this.

Fault: P0118 (coolant temp sensor circuit high). Started with a dead-flat -50°C reading on G62 in the diagnostic software (VCDS/ODIS-style tool), on both data groups showing coolant temp.

What I've tested so far:

- Unplugged G62, tested sensor connector directly: positive pin to battery negative read ~3.5V, negative pin to battery negative showed no continuity.

- Bridged positive to negative on the connector — still -50°C on the laptop.

- Bridged the positive wire straight to battery negative — still -50°C, no change at all.

- Since G62 is a ground-referenced sensor (dedicated return to the ECU, not chassis ground, i belive), I hard-wired a new ground/return wire from the sensor pin straight to the ECU connector, bypassing the harness.

Result: Worked fine initially. After driving a while, values went erratic, then dropped out completely. Stopped the car, values came back roughly normal. Now noticing that revving the engine (even stationary) makes the readings turn to garbage.

New finding: The oil temperature sensor is doing the exact same thing — erratic/garbage readings under revs, fine at idle/stop. Two unrelated sensor circuits failing together under the same conditions.

Checked the alternator: output is sitting at 13V, not climbing with RPM as far as I've tested so far.

Given two separate sensor circuits are misbehaving in sync with engine speed, I'm suspecting a charging system issue (weak alternator, bad diode/ripple, or a poor ground) rather than two coincidental wiring faults. Planning to do an AC ripple test across the battery terminals next.

Has anyone seen this exact combo before — P0118 plus other sensors going haywire under revs — and did it turn out to be the alternator, a ground strap, or something else entirely? Any input on where to focus before I pull the alternator would be appreciated.

At 13V the alternator is barely working at all. If original, the brushes are probably worn to the point of barely making contact.

The arcing and sparking that may be happening between these brushes and the sliprings could be causing various electrical problems.