If a scan reveals that the high pressure switch is open to ground, does that mean without exception that the high pressure switch is faulty? Assuming it's connected.[/align]

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'98 Audi A4 2.8 Quattro 5-sp
TT carriers A8 rotors
VMaxx coilovers
2-piece xenons

No. It could be a bad wire as well. All it means is that the circuit is open somewhere. Break out the DVOM and start doing resistance tests. Or borrow a sensor and see if the fault disappears.

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01 A4 1.8T Quattro Sport
Eibach, Hotchkis, Upsolute, Borla

Is there a trouble shooting sequence that I can access somewhere? Is there a relay in the circuit before the switch that could be at fault? The a/c was working fine, then suddenly, no cold air. I checked the wire and connector and see no obvious issues.

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'98 Audi A4 2.8 Quattro 5-sp
TT carriers A8 rotors
VMaxx coilovers
2-piece xenons

You'll need a wiring diagram, which I do not have.

You can test the switch with a resistance check or jump the connector and see if the fault disappears.

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01 A4 1.8T Quattro Sport
Eibach, Hotchkis, Upsolute, Borla