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
Wire harness defect.
Backflow temperature sensor defect -
happens if sensor is disconnected or short
Outlet temperature sensor fault
(Temperature below 36°F or above 210°F).
Outlet temperature sensor not sensing
expected output temperature.
(Status message, not an error).
* By installer or service technician only.
C 1210 ES/ESC
1. Check continuity of the flue gas limiter (see fig. 76, page 51). 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. 83, #7, page 59.
2. Measure resistance loose on wire harness nearby the overmolding, must be
3. Check diagnostic menu > 1P, "nC" must be showed on the display.
1. Check backflow temperature sensor connection. See fig. 76, #1, page 51.
2. Replace exhaust temperature sensor.
3. Check sensor (ref. page 48, chapter 11).
1. Check wire connection at hot water temperature sensor. See fig. 82, page 58.
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
3. In areas where water has a high mineral content, periodic descaling may
4. Check sensor (ref. page 48, chapter 11).
1. Check wire connection at hot water temperature sensor, see fig. 82, page 58.
2. Check gas pressure. Low gas pressure may prevent the heater from reaching
desired output temperature.
3. Check supply voltage. It must be 120VAC and properly grounded.
4. Possible defective control unit call Bosch Water Heating for further
Problem solving | 43
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