d3 High limit / temperature limiter circuit open

What the control is telling you

The temperature limiter circuit is open or an external limit has tripped, locking the boiler out.

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

Likely causes

• Sensor or lead open or shorted
• Temperature guard TB1 tripped or jumper missing across 8-9 or PR-P0
• External limit, LWCO or pressure device tripped
• Air in the system or low water pressure

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 the sensor and leads for breaks or shorts.
3. If no outside guard device is used, make sure the link across 8-9 or PR-P0 is in place.
4. Reset any external temperature limiter, LWCO or pressure device that is installed.
5. Purge the boiler and check the water pressure (this fault can appear at first start-up if air remains).

Models that show this code

4 models in Bosch Greenstar

Other codes on the same units

• b1 Code plug not detected
• b2/b3/b4/b5/b6 Internal data error
• b7 Burner control fault
• C6 Fan not running
• d5 External supply temperature sensor fault (low-loss header)
• E2 Supply temperature sensor fault
• E3 Additional supply temperature sensor fault
• E9 Safety high limit or flue gas limiter tripped