AIR CONDITIONING (page 5 of 7)

AIR CONDITIONING

DESCRIPTION AND OPERATION

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Diagnostics: The ECU monitors the FBH system for faults. Any faults detected are stored in a volatile memory in the 
ECU, which can be interrogated by Testbook/T4. A maximum of three faults and associated freeze frame data can 
be stored at any one time. If a further fault is detected, the oldest fault is overwritten by the new fault.

The ECU also incorporates an error lockout mode of operation that inhibits system operation to prevent serious faults 
from causing further damage to the system. In the error lockout mode, the ECU immediately stops the FBH fuel pump, 
and stops the combustion air fan and circulation pump after a cool down time of approximately 2 minutes. Error lockout 
occurs for start sequence failures and/or combustion flameouts, heat exchanger casing overheat and out of limit input 
voltage. The error lockout mode can be cleared using Testbook, or by disconnecting the battery power supply for a 
minimum of 10 seconds.

Start Failure/Flameout: If a start sequence fails to establish combustion, or a flameout occurs after combustion is 
established, the ECU immediately initiates another start sequence. The start failure or flameout is also recorded by 
an event timer in the ECU. The event timer is increased by one after each start failure or flameout, and decreased by 
one if a subsequent start is successful. If the event timer increases to three (over any number of drive cycles), the 
ECU enters the error lockout mode.

Heat Exchanger Casing Overheat: To protect the system from excessive temperatures, the ECU enters the error 
lockout mode if the heat exchanger casing temperature exceeds 105 

°

C (221 

°

F).

Out of Limit Voltage: The ECU enters the error lockout mode if the battery or alternator power input is less than 10.5 

±

 0.3V for more than 20 seconds, or more than 15.5 

±

 0.5V for more than 6 seconds.