Code A9 = Heat exchanger overheat. The thermistor in the heat exchanger detected temperature above safe limit. Serious code — won't restart until underlying cause is addressed.
Almost always: scaled heat exchanger
By far the most common cause. Scale insulates the heat exchanger. Burner can't transfer heat fast enough; metal temperature rises until safety trips. Fix: full descale.
Other causes
Restricted combustion air or exhaust
Blocked intake or vent. Inspect and clean.
Low water flow during draw
Clogged inlet filter or partially closed isolation valve. Clean.
Greenstar combi: heating loop issue
If heating-loop circulator pump fails or air-lock develops, hot water can't move away from the exchanger. Verify circulator and purge air.
Reset procedure
- Power off completely; wait 30 minutes to cool
- Power back on, attempt Level 1 reset
- If code returns immediately, do NOT keep resetting
If A9 persists
Stop troubleshooting DIY and call a Bosch-trained installer. Repeated overheat events cause cumulative heat-exchanger damage.
Prevention
- Annual descaling — non-negotiable in hard water
- Water softener above 11 gpg
- Annual inlet filter cleaning
Bottom line
A9 is the cost of skipping annual maintenance. Descale, then call an installer if it returns.