Code 11 = Ignition failure / no flame. The highest-volume Ruud Ultra Tankless error code. Same diagnostic as Rheem Code 11 (same manufacturing).
Diagnostic sequence
1. Gas supply
- Confirm gas valve at unit is fully open
- Test another gas appliance
- Check propane tank level if LP
2. Air in gas line
If gas was recently turned off, purge by running another appliance 5-10 minutes.
3. Gas pressure
Ruud Ultra Tankless needs 4" WC NG / 10" WC LP minimum sustained during fire. Undersized gas line = #1 chronic cause.
4. Flame sensor (#1 hardware cause)
Cleaning with fine sandpaper resolves ~40% of chronic Code 11 cases on units 5+ years old.
5. Igniter
Visual check for blue spark in dark room. No spark = igniter or cable.
6. Gas valve
Less common. Dealer-only replacement.
Reset
- Address suspected cause
- Press On/Off on controller
- If code returns, escalate
Common scenarios
- Code 11 only at high flow: gas line undersized
- Code 11 after install: air in line or undersized line
- Code 11 in old unit (10+ years): flame sensor or igniter end-of-life
Bottom line
Work top-down. Flame sensor cleaning is free and resolves a meaningful fraction.