P0859moderate severityPowertrain
TCM Communication Circuit Low
on 2009 Audi Q7 3.0L Supercharged V6 · AWD
What this code means
The TCM communication circuit voltage is lower than expected. This may prevent proper data transfer between control modules.
Symptoms
- Transmission fault indicator
- Loss of shift control
- Stored communication codes
Common causes
- Short to ground in circuit
- Corroded connectors
- Faulty TCM
- Wiring harness damage
Diagnostic steps
- Retrieve stored trouble codes
- Inspect circuit for short to ground
- Measure circuit voltage
- Verify TCM operation
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