TPS - Throttle Position Sensor ... this one is pretty self-explanatory in that it reports the current throttle position (WOT, idle, etc) to the ECU. If the adjustment is incorrect you may have an erratic or poor idle amongst other issues.
ACV - Air Control Valve ... this dumps air from the air pump into the exhaust for emissions purposes (at least on my car... I'm assuming the ACV here means the same thing).
Thanks for that Robo. Are these things that people would change to get more performance out of the car? I ask this as the person I bought it from, had it serviced twice at one of the performance shops that specialise in WRX's. So I was wondering whether they had been mucking about with it.
You shouldn't change these things, but some people do.
The TPS has to read between certain voltages which the ECU will learn are closed and wide open, say .5-1.5v. If a TPS is removed as replaced improperly it will no longer be aligned to the throttle plate position and may go out of range for open and closed.
Essentially, someone could screw it up if they weren't careful and/or didn't know what they were doing.
Ok thanks for that. The car has been running fine and the service guys didn't pick anything up (Subaru Dealership). Although they did mention whoever drives/drove it are heavy on the brakes as they only had 10K left in them, when there should be another 20-30K