Bosch 940 ES Installation Manual

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6 720 644 930Problem solving49Ionization failure during operation. 1. Check gas type, see Fig. 2 , page 6 .2. Check three wire connections to ignition group on the lowerfront of the heat exchanger are secure.3. Check venting specifications are met. Improper venting maycause unstable burner flames and loss of flame ionization.4. Check that the minimum power fan speed has been adjusted tothe proper value. See page 20.5. Check gas pressure, see section 3.14 , page 33 .6. Check and adjust CO2 readings, see section 6.4, page 40.Internal hardware/software failure. 1. Disconnect power supply cord and check the two wire andground connections on control board as well as ground connectionon heater chassis.2. Pushing the wrong combination of buttons on the control unitcan create confusion among the microprocessors inside. In thiscase, the error code should not happen more than once or twice.Turn off the water heater. Turn water heater back on and tryresetting error code. Use the reset button ( ) to reset any errorcodes.3. Possible defective control unit. Call Bosch Water Heating forfurther instructions.Ionization error at standby. 1. Loose connection to the flame ionization rod. Verify that thethinner wire leading from the control unit is securely connected tothe flame sensor located on the lower front of the heat exchanger.See Fig. 73 , page 53.2. Flame ionization rod or control unit may be damaged. ContactBosch Water Heating for further instruction.Gas leakage error, gas valve circuitnot closing properly.1. Disconnect power supply cord and check wire connections ongas valve and the two connectors on the control board.2. Flow water out of a hot water tap above the minimum activationpoint of 0.5 GPM. Measure voltage at the gas valve wire plugconnection. The voltage should measure 24VDC between the leftpair of wires and 24VDC between the right pair of wires when theunit is operating. If voltage is not proper, contact Bosch WaterHeating for further instruction.3. Gas valve may be defective, contact Bosch Water Heating forfurther instrucation.Display Cause SolutionTable 31* By installer or service technician only.