Bosch 940 ESO Instruction Manual page 42

Problem solving
Display Cause
Cold water temperature sensor
fault.
Backflow temperature sensor over
309°F.
The appliance will close the burner
and lock.
Overheat sensor (ECO) open
circuit
temperatures are detected 220
°C
104
No flame ionization detected with
water flow.
Table 12
* By installer or service technician only.
42
(resets when cooler
°
F/
).
Solution
1. Check connector on wires coming from top of watervalve for a
secure connection. See Fig.
2. Sensor may trip if water temperature drops below 36°F to
protect heater from freezing conditions Any damage due to
freezing conditions is not covered under warranty.
page 45
3. Check sensor (ref.
1. Check combustion air and exhaust vent piping for blockage.
2. Check if secondary fan connections are disconnected.
3. Check vent openings. A blockage may cause this failure.
4. Call Bosch Water Heating for further instruction.
1. Disconnect power supply cord and check wire connections on
overheat sensor (top right of heat exchanger) and the two
connectors on the control board. See Fig.
2. Check white wire connections at overheat sensor. Clean
terminals with an eraser. If badly corroded, replace sensor and
wire harness.
3. Check vent openings. A blockage may cause this failure.
4. In areas where water has a high mineral content, periodic
descaling may necessary, see
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.
1. Verify that all manual gas shut off valves are open.
Fig. 2
2. Check gas type, see
3. Reset error code and open a water tap to cycle the heater in an
effort to purge air on new installations. 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 to ignition group on the lower
front of the heat exchanger are secure.
5. Check gas pressure, see
6. Check vent openings for blockage. Improper venting may cause
premature failure of the flame sensor rod.
7.
Check that the minimum power fan speed has been adjusted to
the proper value. See page
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 or yellow with proper gas
pressure, confirm CO readings per
2
66 47.
, page
chapter 14
, ).
66 47.
, page
chapter 10.3
.
page 6
, .
chapter 7.6 page 25
, .
30.
section 10.4
.
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