Bosch 520PN Applications Manual page 87

Display Cause
Backflow temperature sensor over 309°F.
The appliance will close the burner and lock
(non-volatile).
Overheat sensor (ECO) open circuit (resets
when cooler temperatures are detected
220°F/104 °C).
No flame ionization detected with water
flow.
Ionization failure during operation.
Table 39
* By installer or service technician only.
C 1210 ES/ESC
Solution
1. Check inlet/outlet vent pipes.
2. Check if secondary fan connections are disconnected.
3. Check if venting specifications are met. Long vent lengths, venting with more
than three elbows, blocked vent or combination venting may cause this failure.
See chapter 4.3, page 11.
4. Call Bosch Water Heating for further instructions.
1. Disconnect power supply cord and check wire connections on the overheat
sensor (fig. 76, page 51) and the two connectors on the control board.
2. Check white wire connections at the overheat sensor. Clean terminals with an
eraser. If badly corroded, replace sensor and wire harness.
3. Check that venting specifications are met. Long vent lengths, venting with
more than three elbows, blocked vent or combination venting may cause this
failure.
4. In areas where water has a high mineral content, periodic descaling may
necessary, see chapter 7.3, page 36.
5. Unplug power supply cord to the water heater. Open a hot water tap for
several minutes to allow cold water to pass through heat exchanger. Close hot
water tap and disconnect lead wires to overheat sensor. Using a multimeter,
check continuity through overheat sensor contacts. Replace sensor if open.
1. Verify that all manual gas shut off valves are open.
2. Check gas type. See fig. 2, page 8.
3. Reset error code and open a water tap to cycle the heater in an effort to purge
air. Cycling hot water tap on and off multiple times may be necessary. If heater
still faults with EA error code, have a licensed gas technician properly purge air
out of the gas line leading to the water heater.
4. Check three wire connections on the lower front of the heat exchanger are
secure.
5. Check gas pressure. See chapter 4.14, page 31.
6. Check venting specifications are met. Improper venting may cause premature
failure of the flame sensor rod. See chapter 4.3, page 11.
7. Check that the minimum power fan speed has been adjusted to the proper
value. See page 21.
8. Observe inside the viewing window of the heat exchanger when a hot water tap
is opened. Sparking should be followed by a steady blue flame. If flame is
unstable/yellow with proper gas pressure, confirm CO
7.5, page 37.
1. Check gas type, fig. 2, page 8.
2. Check three wire connections to ignition group on the lower front of the heat
exchanger are secure.
3. Verify that venting specifications are met. Improper venting may cause
premature failure of the flame sensor rod. See chapter 4.3, page 11.
4. Check gas pressure. See chapter 4.14, page 31.
5. Check and adjust CO2 readings. See chapter 7.5, page 37.
6. Check that the minimum power fan speed has been adjusted to the proper
value. See page 21.
Problem solving | 45
readings per chapter
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