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Problem Solving; Error Code Diagnostics - Bosch 940 ESO Manual

940 eso - natural gas / 940 eso - liquefied petroleum (lp) gas
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36 | Problem solving

13
Problem solving
13.1

Error code diagnostics

To remove error code from the display, press the reset
button.
Display
Cause
Fault in the flue gas limiter.
Temperature above 230°F (110 °C)
inside the cabinet.
Wire harness defect.
Backflow temperature sensor defect -
happens if sensor is disconnected or
short circuit.
Outlet temperature sensor fault
(Temperature below 36°F or above
210°F).
(Flashing)
Outlet temperature sensor not sensing
expected output temperature.
(Status message, not an error).
Primary fan rotation too low in
operation.
Table 22
* By installer or service technician only.
6 720 644 956 (2016/05)
Solution
1. Check continuity of the flue gas limiter (see fig. 64, page 42). Go to steps two and
three to determine flue gas limiter fault and repair it.
2. Check for flue gas leakage around the top and bottom seals of heat exchanger, use
mirror to check around the rear as well as the viewing window.
3. Check that flue gas limiter connector and spade connections are secure.
4. Unplug heater and check the wiring harness connections on the control board.
1. Check wire harness, see fig. 71, #7, page 49.
2. Measure resistence loose on wire harness nearby the overmolding, must be 115 -
120 ohm.
3. Check diagnostic menu > 1P, "nC" must be showed on the display.
1. Check backflow temperature sensor connection. See fig. 64, #1, page 42.
2. Replace exhaust temperature sensor.
3. Check sensor (ref. page 40, chapter 15).
1. 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.
2. In areas where water has a high mineral content, periodic descaling may necessary.
3. Check sensor (ref. page 40, chapter 15).
1. Check gas pressure. Low gas pressure may prevent the heater from reaching
desired output temperature.
2. Check supply voltage. It must be 120VAC and properly grounded.
3. Possible defective control unit call Bosch Water Heating for further instructions.
1. Disconnect power supply cord and check wire connection on back side of fan and
the connectors on the control board, see fig. 71, page 49.
2. Check supply voltage. It must be 120VAC and properly grounded.
3. Check venting specifications are met. Long vent lengths, venting with more than
three elbows, blocked vent or combination venting may cause this failure.
4. Ensure intake and exhaust terminations maintain the required clearances stated in
the manual. Cross contamination between intake and exhaust may cause the fan to
alter its rotational speed.
5. Check gas pressure. Low gas pressure may cause the fan to change its speed to
meet desired temperature.
6. Possible defective control unit call Bosch Water Heating for further instructions.
940 ESO

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