check engine light no code on 1999 Volkswagen Beetle

how do i figure out what is wrong with the car when the check engine light is on and there is no code.
took it to auto zone everything came up ok

Asked by for the 1999 Volkswagen Beetle
Generally if the "Check Engine Light" has come on there will be a code stored in the vehicle's computer and it will be stored until erased using a scantool or the car has completed a sufficient number of drive cycles without having seen the problem reoccur. In this case it has done a self test seen no fault and erases the code.
This being said I have seen cases where one scan tool does not see a code and another brand of scan tool sees a code on the same vehicle. We have three scantools where I work.
1 more answer
I have a 1999 plymouth breeze 2.0 and I replaced the ignition coil, changed the spark plugs. Vishually inspected the wires which were fine. When I replaced everything that could cause a misfire, it still came up codes p300 to p304. I found I had to erase the codes one by one. When I accomplished that, the engine light is still on without any link. I have one of the best OBD11 scanners there is and it wold pick up a minor fluxuation in the circuits. My guess is its the ECM or the onboard computer itself. Have it tested at AZ or a garage. If that doesn't sound right, does anyone have any ideas? I'm going through the same thing as you