E9 Safety high limit or flue gas limiter tripped

What the control is telling you

A heat exchanger safety limit or the flue gas temperature limiter has opened. The same code is used when the pump has seized.

Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers

Likely causes

• Faulty limiter or wiring
• Low system pressure or trapped air
• Pump capacitor weak or pump seized
• Blown PCB fuse
• Clogged DHW side of the heat exchanger

Test and repair steps

1. Warning: Disconnect mains power and close the gas cock before opening the appliance or working on the gas train.
2. Check both the flue gas limiter and heat exchanger limiter and their leads.
3. Check the system operating pressure and purge the appliance.
4. Check the pump capacitor and replace the pump if bad.
5. Check the PCB fuse (6.3 A per IEC) and replace if blown.
6. Check the DHW side of the heat exchanger and replace if blocked.

Models that show this code

4 models in Bosch Greenstar

Other codes on the same units

• d3 High limit / temperature limiter circuit open
• d5 External supply temperature sensor fault (low-loss header)
• E2 Supply temperature sensor fault
• E3 Additional supply temperature sensor fault
• F0 Internal fault
• F1 Internal data error
• F7 Flame detected with burner off
• Ad DHW tank temperature not detected