Officers say auto thefts in the city can increase up to 30 percent in November and December compared to the average amount the rest of the year.
Fresno County's HEAT task force spent the early morning hours of Tuesday looking for cars warming up without a driver.
With temperatures dropping significantly this time of year, officers say it's the most opportunistic time for would-be thieves to prey on their victims.
"Rather than chasing bad guys this morning, we're out there contacting these people that, maybe unaware of how easy their vehicles are stolen and how quickly it takes," Sgt. Leonard Sherman said.
So how fast and easily can a car thief get into an idling car? Officers say it can take just seconds.
At one location in Central Fresno, not one but two cars were warming up with lights on in one driveway, completely unattended. Although there was a gated fence, the owners left it unlocked.
Undercover officers waited three minutes before knocking on the door and urging the owners not to leave their cars alone.
They did the same thing educating people at a 17 other locations in Fresno over a two-hour period.
Each time owners neglected their vehicles.
Officers say losing your car will end up costing you even if you get it back.
"When your vehicle is stolen even if you do get it back you may have tow fees. You may have damage you need to repair. So you're looking at, even if you have insurance you have deductibles. Best case scenario your out of pocket close to $1,000 dollars. And that's very common," Sherman said.
Mechanics at D & R Automotive in Downtown Fresno say in many cases you don't need to warm up your engine.
"All the newer vehicles from 1996 up you don't really need to because of all the electronics and stuff on them. I think it's more of a convenience for the customers that want the car warmed up when they get in," Tony Dellamaggoire said.
Cars more than 15 years old will most likely need to be warmed up to avoid engine failure while driving.
But experts say you should never leave your car alone, even for just a minute.