The public got a “ticket-fixing 101’’ lesson Monday in the Bronx case of an NYPD lieutenant being tried on charges she leaked information to colleagues about the scandal that ensnared 16 cops.

On the fourth day of the non-jury trial of Lt. Jennara Cobb, who’s accused of divulging details of the Internal Affairs Bureau’s investigation in 2011, Detective Randy Katakofsky, the investigator who first went to Bronx prosecutors with the case, testified about all the different ways a summons can get tossed.

“The officer can just tear it up and throw it away,” or change information on the ticket to disassociate it from the offender, said Katakofsky, who worked in IAB with Cobb.

Another way to quash a ticket, he said, “Phone calls could be made to take it out of the summons box” in the precinct. Or, “if you were able to reach the mail guy in the borough [headquarters], he could tear it up,” the cop added.

But once a ticket reached 1 Police Plaza, it was tougher to get it nixed, according to Katakofsky, who was slapped with departmental charges for tipping off Cobb to the investigation.

The solution, then?

“The [ticket-issuing] officer would commit perjury and not testify properly,” he said.

Katakofsky also said once word got out about the probe, ticket-fixing quickly stopped or became “sporadic.”

Sources say the credibility of the case rests with Katakofsky, 36, who The Post reported, turned in early retirement papers last month for posttraumatic stress disorder. If the embattled cop — who has been lambasted by police-union officials and the rank-and-file for turning in fellow cops over the once-standard practice — is found to be mentally unsound, it could affect the outcome of the large-scale case.

Katakofsky’s testimony continues Tuesday.

Meanwhile on Monday, prosecutors also played several wiretap recordings that they claim showed a handful of cops charged in the sweeping scam tried to cover their tracks after Cobb allegedly leaked the information.

In a Feb. 5, 2010, call, the DA charges, Officer Viriglio Bencosme told Officer Luis Rodriguez, “Don’t take care of summonses on the phone anymore. IAB is recording or whatever, and it’s not worth taking the chance.”

Cobb, the first of the so-called “tix-fix” cops to be tried, faces up to a year in prison if convicted.