Bosch 940 ESO Manual page 38

38 | Problem solving
Display Cause
No flame ionization detected with
water flow.
Ionization failure during operation.
Internal hardware/software failure.
Ionization error at standby.
Gas leakage error, gas valve circuit not
closing properly.
Table 22
* By installer or service technician only.
6 720 644 956 (2016/05)
Solution
1. Verify that all manual gas shut off valves are open.
2. Check gas type. See fig. 2, page 8.
3. Reset error code and open a water tap to cycle the heater in an effort to purge air.
Cycling hot water tap on and off multiple times may be necessary. If heater still faults
with EA error code, have a licensed gas technician properly purge air out of the gas
line leading to the water heater.
4. Check three wire connections on the lower front of the heat exchanger are secure.
5. Check gas pressure. See chapter 8.6, page 23.
6. Check venting specifications are met. Improper venting may cause premature
failure of the flame sensor rod. See chapter 5, page 11.
7. Check that the minimum power fan speed has been adjusted to the proper value.
See page 28.
8. Observe inside the viewing window of the heat exchanger when a hot water tap is
opened. Sparking should be followed by a steady blue flame. If flame is unstable/
yellow with proper gas pressure, confirm CO2 readings per chapter 11.4, page 30.
1. Check gas type, fig. 2, page 8.
2. Check three wire connections to ignition group on the lower front of the heat
exchanger are secure.
3. Verify that venting specifications are met. Improper venting may cause premature
failure of the flame sensor rod. See chapter 5, page 11.
4. Check gas pressure. See chapter 8.6, page 23.
5. Check and adjust CO2 readings. See chapter 11.4, page 30.
6. Check that the minimum power fan speed has been adjusted to the proper value.
See page 28.
1. Disconnect power supply cord and check the two wire and ground connections on
control board as well as ground connection on heater chassis.
2. Pressing the wrong combination of buttons on the control unit can create confusion
among the microprocessors inside. In this case, the error code should not happen
more than once or twice. Turn off the water heater. Turn water heater back on and try
resetting error code. Use the reset button (
3. Possible defective control unit call Bosch Water Heating for further instructions.
1. Loose connection to the flame ionization rod. Verify that the thinner wire leading
from the control unit is securely connected to the set of electrodes located on the
lower front of the heat exchanger.
2. Flame ionization rod or control unit may be damaged. Contact Bosch Water Heating
for further instruction.
1. Disconnect power supply cord and check wire connections on gas valve and the
two connectors on the control board.
2. Flow water out of a hot water tap above the minimum activation point of 0.5 GPM.
Measure voltage at the gas valve wire plug connection. The voltage should measure
24VDC between the left pair of wires and 24VDC between the right pair of wires when
the unit is operating. If voltage is not proper, contact Bosch Water Heating for further
instruction.
3. Gas valve may be defective, contact Bosch Water Heating for further instruction.
) to reset any error codes.
940 ESO