the WHCU at the same time for more than 2 seconds to erase the fault history.
 (4)  If only the green "BIT TEST OK" LED comes ON, then there was an intermittent fault.
 (a)  Press the "BIT LAMP RESET" and the "LAMP TEST" switches on the WHCU at the same time for more than 2 seconds to erase the fault history.
 B.  Procedure
 (1)  If the red "WHCU-LRU" LED is ON, then do these steps:
 (a)  Replace the WHCU (AMM 30-41-11/401).
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 (2)

(3)

(4)

(5)

(6)

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737-300/400/500MAINTENANCE MANUAL
If the red "BUS POWER" LED is ON, then do these steps:
 (a)  
Check for 115V AC bus power at the connector at the back of the WHCU (WDM 30-41-11, WDM 30-41-12).

 (b)  
If there is not 115V AC at the connector, then repair the

 wiring.If the red "WINDOW POWER" LED is ON, then do these steps:

 (a)  
Check the resistance of the window (AMM 30-41-21/501).

 (b)  
If the resistance of the window is not within tolerances, replace the window (AMM 56-11-11/401; AMM 56-12-11/401).

 (c)  
If the resistance of the window is within tolerances, then do a check of the heater wiring between the WHCU and the window (WDM 30-41-11, WDM 30-41-12).

 (d)
If you find a problem with the wiring, repair it.If the red "WINDOW SENSOR" LED is ON, then do these steps:

 (a)  
Check for 290-358 ohms at the sensor (WDM 30-41-11, WDM 30-41-12).

 (b)  
If there is not 290-358 ohms at the sensor, then disconnect the sensor and connect the spare sensor. 1) If the spare sensor is already connected, then replace the

 window (AMM 56-11-11/401; AMM 56-21-11/401).

 (c)  
If there is 290-358 ohms at the sensor, then do a check of the sensor wiring between the WHCU and the window (WDM 30-41-11, WDM 30-41-12).

 (d)
If there is a problem with the wiring, repair it.If the "P5-9 CONTROL POWER" LED is ON, then do these steps:

 (a)  
Make sure that the P5-9 CONTROL POWER switch on the P5-9 module is set to POWER (WDM 30-41-11, WDM 30-41-12).

 (b)  
Do a check of the power wiring from the P5-9 module to the WHCU (WDM 30-41-11, WDM 30-41-12).

 (c)
If you find a problem with the wiring, repair it.
If the red "WHCU-LRU" and "WINDOW POWER" LEDs are ON, then do these
 steps:

 (a)  
Check the resistance of the window (AMM 30-41-21/501).

 (b)  
If the resistance of the window is not within tolerances, replace the window (AMM 56-11-11/401; AMM 56-12-11/401).

 (c)  
If the resistance of the window is within tolerances, then do a check of the heater wiring between the WHCU and the window (WDM 30-41-11, WDM 30-41-12).

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 (7)  (d) If you find a problem with the wiring, repair it.If all five red LEDs are ON, then do these steps: (a) Make sure the POWER CONTROL switch on the P5-9 module is set to POWER.
 (8)  If some other combination of red LEDs are ON, then check the Bit Indication Matrix in the figures to determine the failure mode.

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737-300/400/500MAINTENANCE MANUAL
PREREQUISITES
ELECTRICAL POWER (AMM 24-22-00/201) THE OVERHEAT LIGHT
　
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