IIRC, he was stopped for a missing rear license plate—the reason being, it was blown away by the explosion.
From a little research:
The FBI initially stated that they thought the license plate had blown off/been loosened by the explosion and asked for help searching for it. But the prosecutor in his closing argument stated that the evidence showed that McVeigh had removed it before the explosion.
Apparently the plate came from another car that McVeigh had owned, which broke down.
The car was parked far enough away that the explosion would not have blown parts off of it.
McVeigh's motivation for removing the plate may have just been to prevent anyone from tracking the origin of the car while it was parked in Oklahoma City for several days. He may have figured that after the explosion the police would be headed in the opposite direction. But since it was not a reliable vehicle, making a getaway doesn't seem to have been a top concern.