50 | Problem solving
Error code diagnostics
To remove error code from the display, press the reset
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
Outlet temperature sensor fault
(Temperature below 36°F or above
Heat exchanger (temperature sensor
Outlet temperature sensor not sensing
expected output temperature.
(Status message, not an error).
Bypass valve solenoid damaged or
* By installer or service technician only.
6 720 805 440 (2014/11)
1. Check continuity of the flue gas limiter (see fig. 80, page 57). 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. 87, #7, page 65.
2. Measure resistance on wire harness nearby the rubber grommet, 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. 80, #1, page 57.
2. Replace exhaust temperature sensor.
3. Check sensor (ref. page 55, chapter 11).
1. Check wire connection at hot water temperature sensor. See fig. 86, page 64.
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.
3. In areas where water has a high mineral content, periodic descaling may necessary.
4. Check sensor (ref. page 55, chapter 11).
1. Check wire connection at hot water temperature sensor. See fig. 82, page 59.
2. In areas where water has a high mineral content, periodic descaling may necessary.
3. Check sensor.
1. Check wire connection at hot water temperature sensor, see fig. 86, page 64.
2. Check gas pressure. Low gas pressure may prevent the heater from reaching desired
3. Check supply voltage. It must be 120VAC and properly grounded.
4. Possible defective control unit call Bosch Water Heating for further instructions.
1. Check wire connection at the bypass valve, fig. 86, page 64.