Q: CEL lights p0501 p0500 p1529 u1400 on 2001 Hyundai Elantra

Codes P0500, P0501, and P1529 are related to vehicle speed sensor input. The engine control unit relies on sensor input from the wheels and transmission - as well as dozens of other sensors - to determine how fast the vehicle is going. If the check engine light is on with these codes present, the ecu will put the vehicle into sort of a "limp mode" where it will run, but not run correctly. Possible causes could be a wheel speed sensor which also provides input for the anti-lock braking system if equipped (most commonly at the right front wheel), or the vehicle speed sensor which is typically located on or in the transmission. Code P1400 is related to exhaust gas recirculation (egr). Through the egr valve a small amount of burned air and fuel is rerouted from the exhaust and back into the combustion chambers at certain speeds to cool the combustion chambers and reduce the chances for ’knock and ping". Although unrelated to vehicle speed, this code may simply be present due to the way the engine is running due to the speed codes. Have a certified technician inspect the vehicle. If replacement of a wheel or vehicle speed sensor is determined to be the cause and replaced, the egr code may go away. If it returns, then the egr system will have to be inspected to find the exact cause for that code.