Problem Solving; Error Code Diagnostics - Bosch 940 ES Installation And Operating Instructions Manual

Problem solving

9 Problem solving
9.1

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.
60
Solution
1. Check continuity of the flue gas limiter (see fig. 67, page 67). 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. 74, #7, page 74.
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. 67, #1, page 67.
2. Replace exhaust temperature sensor.
3. Check sensor (ref. page 64, chapter 11).
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 64, chapter 11).
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. 74, page 74.
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 ES – 6 720 644 930 (2019/09)