Fault Code Lookup

HVAC Sales Master
9
Open Inducer Limit switch
This code typically points to
  • Open inducer limit switch

Next checks to confirm
  1. Check inducer limit continuity
  2. Public abnormal-condition text also points to variable-speed draft inducer troubleshooting manual

Escalation
Heating interrupted
Reset behavior
Device must cool/clear.
Urgent
Fix now
Cross-generation conflict
Code 9 means completely different things across Trane/American Standard legacy platforms. On UD2/DD2 and UC1/DC1, it's an ignitor/grounding fault. On AUY/TUY and AUH2/TUH2, it's an open inducer limit — totally different diagnostic path. Your model prefix determines which it is.
Source
Trane 18-CD20D3-21 Installer's Guide, p.35-36