AU-C Section 200.29: Failure to Achieve an Objective

AU-C Section 200.29 says:

"If an objective in a relevant AU-C section cannot be achieved, the auditor should evaluate whether this prevents the auditor from achieving the over-all objectives of the auditor and thereby requires the auditor, in accordance with GAAS, to modify the auditor's opinion or withdraw from the engagement (when withdrawal is possible under applicable law or regulation). Failure to achieve an objective represents a significant finding or issue requiring documentation in accordance with section 230, Audit Documentation."

Whether or not an audit objective has been achieved depends on the Auditor's professional judgment, and might include an analysis of 

• the results of audit procedures performed in complying with the requirements of GAAS;
• the auditor's evaluation of whether sufficient appropriate audit evidence has been obtained; and
• whether more needs to be done in the particular circumstances of the audit to achieve the objectives stated in GAAS.  

Circumstances that might prevent the Auditor from achieving an objective include those that:

•  prevent the auditor from complying with the relevant requirements of an AU-C section.
• result in it not being practicable or possible for the auditor to carryout the additional audit procedures or obtain further audit evidence (e.g., due to a limitation in the available audit evidence) 

The Auditor doesn't necessarily have to document that he achieved the objectives (e.g., in a separate checklist), but his documentation of conclusions in various audit areas should provide evidence that the objectives were achieved. 

https://www.aicpa.org/content/dam/aicpa/research/standards/auditattest/downloadabledocuments/au-c-00200.pdf