Fault Code Lookup

HVAC Sales Master
6
115 Volt AC power reversed or Poor Grounding
This code typically points to
  • Reversed polarity
  • Poor grounding

Next checks to confirm
  1. Verify line polarity and grounding integrity

Escalation
Safety/control fault
Reset behavior
Clears when wiring/ground issue 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-CD19D2-18 Installer's Guide, p.23