The set of ‘crystals’ that go into the transmitter and receiver, do you have them in correctly? Tx in radio/controller, Rx in the receiver/car.

What numbers are they? Any labels on the radio? As far as I know 75MHz ones are interchangeable, I seem to remember that not all 27MHz sets are by brand. Need to match the radio, 75 crystals don’t work in a 27 radio for example.

They occasionally go bad or get damaged, see if you can borrow a set to try. Or buy a set, they aren’t very expensive. Good batteries in the radio and car right?