'The Wonder Years' Star Busted for DUA ... Ambien

"The Wonder Years" star Jason Hervey just learned the hard way ... never take Ambien before you get behind the wheel.

Tennessee law enforcement tells TMZ ... they got a call about a swerving pickup truck and were able to locate it. Cops say Hervey was behind the wheel and totally out of it ... slurring, red-eyed and confused.

Cops say Hervey miserably failed a field sobriety test. They searched the pickup and found a loaded handgun, but the actor had a permit. Hervey was booked for DUI and carrying a weapon while under the influence.

The incident went down back in January 2015 but it went unnoticed, until this past weekend when Hervey served 48 hours behind bars for a misdemeanor DUI charge. The weapons charge was dropped.

Jason, who played Wayne Arnold on the show, tells us he has a prescription for Ambien and stupidly took one before hopping in the truck for a quick errand, he had no alcohol in his system ... he's thankful cops got him off the road before anything really bad happened.