Fault Code Lookup

HVAC Sales Master
6
Line Reverse Polarity
This code typically points to
  • Reverse polarity

Next checks to confirm
  1. Verify incoming line polarity

Escalation
Control/ignition fault
Reset behavior
Clears when polarity corrected.
Urgent
Fix now
Cross-generation conflict
Code 6 on your Trane/American Standard furnace means different things depending on platform. On UD2/DD2 it's polarity or grounding. On UC1/DC1 it adds 'low voltage'. On AUY/TUY it also adds 'ignitor triac fault'. On AUH2/TUH2 it's only reverse polarity. Your model prefix determines the exact diagnosis.
Source
Trane 18-CD26D1-13 Installer's Guide, p.43