BACnet has various different types of fault algorithms. Each of them is implemented by a different module.

ASHRAE 135:

Certain object types may optionally support a fault algorithm which has externally visible inputs and is performed as part of the object's reliability-evaluation process. This clause defines the standard fault algorithms. To determine which algorithm is applied by which object type, see the object type definitions in Clause 12.

Fault algorithms monitor a value and evaluate whether the condition for transition of reliability exists. The result of the evaluation, indicated to the reliability-evaluation process, may be a transition to a new reliability, a transition to the same reliability, or no transition. The final determination of the Reliability property value is the responsibility of the reliability evaluation process and is subject to additional conditions. See Clause 13.2. Each of the fault algorithms defines its input parameters, the allowable reliability values, and the conditions for transitions between those values. When evaluating the monitored value, all conditions defined for the algorithm shall be evaluated in the order as presented for the algorithm. Some algorithms specify optional conditions, marked as "Optional:" whether or not an implementation uses these conditions is a local matter. If no condition evaluates to true, then no transition shall be indicated to the reliability evaluation process.