Bosch C 950 - Problem Solving; Error Code Diagnostics

Problem solving
C 950/C 1050 (Pro TL 175 C/199 C) – 6 720 805 440 (2019/10)
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9Problem solving
9.1 Error code diagnostics
To remove error code from the display, press the reset button.
Display Cause Solution
A2 Fault in the flue gas limiter.
Temperature above 230°F
(110 °C) inside the cabinet.
1. Check continuity of the flue gas limiter (see fig. 80, page 74). 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.
A3 Wire harness defect. 1. Check wire harness. See fig. 87, #7, page 84.
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.
A4 Backflow temperature sensor
defect - happens if sensor is
disconnected or short circuit.
1. Check backflow temperature sensor connection. See fig. 80, #1, page 74.
2. Replace exhaust temperature sensor.
3. Check sensor (ref. page 71, chapter 11).
A7 Outlet temperature sensor fault
(Temperature below 36°F or
above 210°F).
1. Check wire connection at hot water temperature sensor. See fig. 86, page 82.
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 71, chapter 11).
A8 Heat exchanger (temperature
sensor defect).
1. Check wire connection at hot water temperature sensor. See fig. 82, page 76.
2. In areas where water has a high mineral content, periodic descaling may
necessary.
3. Check sensor.
(Flashing)
A9
Outlet temperature sensor not
sensing expected output
temperature.
(Status message, not an error).
1. Check wire connection at hot water temperature sensor, see fig. 86, page 82.
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 instructions.
C5 Bypass valve solenoid damaged
or disconnected.
1. Check wire connection at the bypass valve, fig. 86, page 82.