Top car Scotty Kilmer has warned and motorists are at risk of being "scammed" by dodgy garages.
The leading expert revealed some firms may try and con road users to splash out for parts they don't need by fooling unsuspecting customers.
He demonstrated that changing over a car's spark plug wires will
This will make it seem as if the vehicle is damaged which could encourage motorists to on buying extra components.
Scotty said: "Today I'm going to show you a common scam that mechanics can do to try and sell you stuff.
"These are spark plug wires. They have to be in order but a mechanic, they can take them out and switch them over.
"To you it might look okay but look what it does to the engine. It'll make it misfire.
"So if you suspect that, get your phone, 'firing line 19964 Celica', and there it is.
"You can see those two spark plug wires are switched so put them back. Knowledge is power."
Spark plugs that are either worn, damaged or installed incorrectly could be the cause of the misfiring.
Driving with a misfiring engine is considered to be dangerous and can lead to road users picking up serious engine damage.
Motorists were quick to praise Scotty for unveiling the little-known secret issue while some individuals admit to having been previously inflicted by the hack.
Viewer @DanaDanarosana said: "Crooked dealer did this to my parents in the '60s... told them their Corvair was worth nothing for trade. Dad figured it out right away, switched them back, and drove away."
@actionjackson3162 remarked: "I saw this happen to a friend's Rx 7 from his sales lot. I think someone was trying to get the car for a much cheaper price.
"But I came along and noticed two of [the] wires seem oddly out of place lengthwise. And that was the problem."
@Jimmyk77 added: "Yeah, an Unscrupulous mechanic did that to me, he switched my ignition coils around on my Tundra 4.7."
