Bosch 940 ESO Instruction Manual page 43

Display Cause
Ionization failure during operation.
Internal hardware/software failure.
Ionization error at standby.
Gas leakage error, gas valve circuit
not closing properly.
Table 12
* By installer or service technician only.
6 720 644 956
Solution
Fig. 2
1. Check gas type, see
2. Check three wire connections to ignition group on the lower
front of the heat exchanger are secure.
3. Check vent openings for blockage. Improper venting may cause
unstable burner flames and loss of flame ionization.
4.
Check that the minimum power fan speed has been adjusted to
the proper value. See page
5. Check gas pressure, see
6. Check and adjust CO readings, see
2
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. Pushing 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 flame sensor located on the lower front of the heat exchanger.
66 47.
See Fig. , page
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 instrucation.
Problem solving
page 6
, .
30.
section 7.6 page 25
, .
section 10.4, page 34.
) to reset any error
43