Code A1 = Ignition failure / no flame. The unit attempted to fire and failed to detect flame. The highest-volume Bosch error code.
Diagnostic sequence
1. Gas supply
- Confirm the gas valve at the unit is fully open
- Test another gas appliance — if all are out, utility supply issue
- Check propane tank level if LP
2. Air in the gas line
If gas was recently turned off, the line has air. Purge by running another gas appliance for 5-10 minutes.
3. Gas pressure
Bosch needs 4" WC inlet on natural gas, 10" WC on propane, sustained during fire. Undersized gas line is the #1 chronic cause. See install guide.
4. Flame rod (#1 hardware cause)
The flame rod detects flame via micro-current. Carbon buildup drops the signal below threshold. Cleaning the rod with fine sandpaper resolves about 40% of chronic Code A1 cases. See flame rod.
5. Igniter
Direct spark igniter wears out. Check for spark when the unit attempts to fire (dark room, sight glass). No spark = igniter. See igniter.
6. Gas valve
Less common. Dealer-only replacement.
Reset procedure
- Address the suspected cause
- Press Reset on the controller
- If code returns immediately, escalate to next cause
Bottom line
Work the diagnostic sequence top-down. Flame rod cleaning is free and resolves a meaningful fraction. Persistent code = call Bosch support at 1-866-642-3198.