Bosch 940 ESO Manual page 37

Display Cause
No rotational speed sensor signal from
primary fan.
(Flashing) Water flow signal over specified
maximum value.
Water flow > 10 gallon/min.
(Status message).
Over-temperature detected by outlet
temperature sensor.
Temperature > 185°F
Cold water temperature sensor fault.
Backflow temperature sensor over
309°F.
The appliance will close the burner and
lock (non-volatile).
Overheat sensor (ECO) open circuit
(resets when cooler temperatures are
detected 220°F/104 °C).
Table 22
* By installer or service technician only.
940 ESO
Solution
1. Disconnect power supply cord and check wire connections on back side of fan and
the two connectors on the control board, see fig. 71, page 49.
2. Check supply voltage. It must be 120VAC and properly grounded.
3. Possible defective component in fan or defective control unit call Bosch Water
Heating for further instructions.
1. Disconnect power supply cord and check wire connections on water valve and the
two connectors on the control board, see fig. 71, page 49.
2. Excessive water pressure and flow. Ensure water pressure is less than 150psi and
flow rate is below 10 gallons per minute.
1. In areas where water has a high mineral content, periodic descaling may be
necessary. See chapter11.3, page 30 for directions.
2. Check sensor (ref. page 39, chapter 14).
1. Check connector on wires coming from top of water valve for a secure connection.
See fig. 64, page 42.
2. Sensor may trip if water temperature drops below 36°F (2°C). Protect heater from
freezing conditions as any damage due to freezing conditions is not covered under
warranty.
3. Check sensor (ref. page 40, chapter 15).
1. Check inlet/outlet vent pipes.
2. Check if secondary fan connections are disconnected.
3. Check if venting specifications are met. Long vent lengths, venting with more than
three elbows, blocked vent or combination venting may cause this failure.
4. Call Bosch Water Heating for further instructions.
1. Disconnect power supply cord and check wire connections on the overheat sensor
(fig. 64, page 42) and the two connectors on the control board.
2. Check white wire connections at the overheat sensor. Clean terminals with an
eraser. If badly corroded, replace sensor and wire harness.
3. Check that venting specifications are met. Long vent lengths, venting with more
than three elbows, blocked vent or combination venting may cause this failure.
4. In areas where water has a high mineral content, periodic descaling may necessary,
see chapter 11.3, page 30.
5. Unplug power supply cord to the water heater. Open a hot water tap for several
minutes to allow cold water to pass through heat exchanger. Close hot water tap and
disconnect lead wires to overheat sensor. Using a multimeter, check continuity
through overheat sensor contacts. Replace sensor if open.
Problem solving | 37
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