Scenes of crime officer Stephen Beattie worked for Northumbria, Staffordshire and Cleveland forces before being suspended in 2011 and later resigning.
More than 350 of his cases, including suspicious deaths and arsons, have been reviewed.
The Crown Prosecution Service has said no criminal charges will be brought.
Both Cleveland Police, which Mr Beattie joined in 2002, and Staffordshire, where he had worked since 1996, investigated him with the Independent Police Complaints Commission (IPCC) managing the inquiries.
The reports have now confirmed Mr Beattie:
But the reports concluded that none of his actions significantly impacted the results of his investigations.
IPCC commissioner Cindy Butts said: "Stephen Beattie repeatedly lied about his qualifications and made judgements in cases that were both outside his remit and his level of expertise.
"Cleveland Police and Staffordshire Police have conducted meticulous investigations examining hundreds of cases and we know that, thankfully, his poor performance did not have a significant impact on the cases he was involved in."
Mr Beattie did receive an official caution in relation to a fraud offence.
Cleveland Police concluded that if he was still employed he would have a case to answer for gross misconduct while Staffordshire concluded he would have a case to answer for misconduct.
A police employee lied about his qualifications, mishandled evidence and was generally poor at investigations, two reports have found.