Three months ago, mayoral challenger Garry McCarthy accused Mayor Rahm Emanuel of being asleep at the wheel when it comes to stopping Chicago’s epidemic of carjackings, waking up only after the problem was out of control.

Now, the embattled mayor is turning to technology — again — to shake that reputation and show he’s on top of the frightening problem.

City Hall disclosed Thursday it’s been quietly using license plate recognition technology on the 26 vans used by Department of Finance crews to put an immobilizing “boot” on vehicles owned by scofflaws with outstanding parking tickets or outdated city stickers.

Cameras are mounted on both sides of the vans; each van can collect about 3,200 license plates per shift. The plate information is fed to databases on stolen cars and other crimes maintained by the Office of Emergency Management and Communications.

Although the Emanuel administration waited until Thursday to publicize the change, the vans have been used in the carjacking crackdown for a few months.

Since March, boot crews have “identified and reported to CPD over 190 hits from the stolen vehicle list,” City Hall said.

“By harnessing an existing city process, we are able to more quickly identify stolen vehicles on streets throughout Chicago, returning them to their owners and preventing these vehicles from being involved in other violent crimes,” Emanuel was quoted as saying in a press release.

Police Superintendent Eddie Johnson added: “With the booter cameras acting as our additional eyes on the ground, we hope to use this data as a tool to expedite our efforts in apprehending and prosecuting car thieves and other violent offenders.”

Last week, a 23-year-old man allegedly shot his brother, wounded two other people, carjacked four drivers and tried to pull off two more carjackings before being arrested in the Gold Coast. It happened over a harrowing two-hour period during the height of the evening rush.

Earrious Moore faces up to 25 years in prison after being charged with one felony count of attempted carjacking for the attempted carjacking of an 84-year-old man. The punishment is more severe than a state penalty.

The elderly victim was shot and grazed by a bullet after Moore allegedly tried to steal the man’s Mercedes.

Johnson pressured federal prosecutors to file charges stemming from the crime spree in Chicago’s marquee shopping district to send a strong message that carjackings in Chicago would not be tolerated.

In February, the Sun-Times reported that dozens of juveniles were charged in Chicago with pointing guns at motorists and stealing their cars, but few were detained longer than a day.

Earlier this week, the Illinois Senate approved legislation designed to close that revolving door through which carjacking suspects often return to the street within 24 hours of their arrests.

In addition to making greater use of technology and pushing for tougher state laws, Chicago Police have joined forces with the FBI, ATF, Illinois State Police and local and federal prosecutors on a team to combat carjackings.

McCarthy has called that too little, too late.

“You don’t wait until you’re up by like 1,000 carjackings to come up with a plan. If I was superintendent, we would already be working with the feds doing federal prosecutions. There’s also a product out there … where you can get a DNA sample returned in 90 minutes. These are no-brainers,” McCarthy told the Sun-Times in February.

“You get the U.S. attorney to make some cases on these guys if they’re conspiracies. If they’re not, you could still take those cases federally. We did this in Newark when I was there, and we broke it immediately. It didn’t take more than a month.”

Pressed to explain the epidemic of carjackings, the fired Chicago police superintendent said, “Criminals are getting released immediately after arrest. Many times, they’re not being prosecuted. If there’s no sanction, what the hell? If you want to go take somebody’s Porsche, go do it.”