Bosch 940 ES Installation And Operating Instructions Manual page 62

Indoor model natural gas/ liquefied petroleum (lp) gas
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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 41 * By installer or service technician only.
62
Solution
1. Verify that all manual gas shut off valves are open.
2. Check gas type. See fig. 2, page 12.
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 4.14, page 45.
6. Check venting specifications are met. Improper venting may cause premature
failure of the flame sensor rod. See chapter 4.3, page 15.
7. Check that the minimum power fan speed has been adjusted to the proper
value. See page 30.
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 7.4, page 52.
1. Check gas type, fig. 2, page 12.
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 4.3, page 15.
4. Check gas pressure. See chapter 4.14, page 45.
5. Check and adjust CO2 readings. See chapter 7.4, page 52.
6. Check that the minimum power fan speed has been adjusted to the proper
value. See page 30.
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 (
codes.
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
940 ES – 6 720 644 930 (2019/09)

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