Professional due care means performing the job or assignment by using all your knowledge and following all standards or procedures required to do the job or assignment at the best level.
In auditing professional due care means performing the audit engagement by following all required audit standards and law and regulation. So the auditor is not only required to possess updated qualification on auditing but also he is required to carry out audit work by following all procedures required to perform the work diligently.
If the auditor perform the audit work without professional due care, he may end up reaching wrong conclusion with regard to fairness and truth of financial statements. Also performing audit without due professional care may lead to audit liability which may lead to financial loss to audit firm.
To maintain due professional care auditor need to update his knowledge and techniques also need to make sure that he follows all standard, laws and regulation required in his audit engagement.