I had a Audi A4 1800 who gave me the following diagnose faults: 8001/P1593/005523 – Altitude Adaptation: Signal Out of Tolerance Possible Symptoms Unknown Commentary. This is one of those fault codes where the ECU knows something is amiss, but cannot pinpoint the cause.
Possible Causes; MAF sensor (G70) out of tolerance.
Leak in intake system.
Throttle position sensors (G187/G188) out of tolerance.
Check MAF/replace MAF Sensor (G70).
Check intake systems for leaks and repair.
Check Throttle position sensors (G187/G188).
As you can see the Ecu cannot pin point the problem, But what I have done is I have replaced the MAF sensor and this cure the problem.