Diagnostics

Diagnostics
The ECU in the FBH unit monitors the system for faults. Any faults detected are stored in a volatile memory in the the ECU, which can be interrogated by Testbook. 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 degrees C (221 degrees F).

Out of limit voltage. The ECU enters the error lockout mode if the battery or alternator power input is less than 10.5 ± 0.3 V for more than 20 seconds, or more than 15.5 ± 0.5 V for more than 6 seconds.