After doing a little research (I own a couple of pretty decent scanning tools and have become familiar with how they function)
Without knowing the actual code for the error there is no way of telling exactly what it's showing as the error. The Code is the PXXXX part of the error that the scanner will detect. There are a number of errors that will make the check engine light come on like that, some are as easy as making sure your gas cap is on tight. Others may show the same exact Gas cap code but for a malfunctioning Evap system. So the codes may be interpreted in many different ways, sometimes they will actually mislead you into buying parts that are not needed.
But fist you need to know the exact number of the code and not the name given to it. Then the circumstances when it happens (as you have already stated), Any effects that it might have on your car as far as how is runs or drives since the code has been showing. Also think back to anything that was touched prior to the codes showing, even filling up with gas, checking you oil etc. These will be places to look for a loose wire or possible connection with the time frame of when it all started.
This is the closest answer I could find:
Vehicle Speed Sensor "A" Circuit Low Input
What does this mean?
OBD Code P0502refers to
The vehicle speed sensor outputs a 4 pulse signal for every revolution of the rotor shaft, which is rotated by the transmission output shaft via the driven gear. After this signal is converted into a more precise rectangular waveform by the waveform shaping circuit inside the combination meter, it is then transmitted to the ECM. The ECM determines the vehicle speed based on the frequency of these pulse signals.
Possible sumptoms of OBD code P0502
- Engine Light ON (or Service Engine Soon Warning Light) - Speedometer may not work properly - Transmission shifting problem
Possible causes of OBD code P0502
- Vehicle speed sensor harness or connectors is open or shorted - Vehicle speed sensor - Combination meter - Faulty ECM. The Error code is generally activated on detection of the following conditions: The signal sent by the vehicle speed sensor to ECM is too low.
*You can use this to try to figure out what may be part of the problem but remember that the codes don't always point to the actual problems. They just let you know that while performing a test that test either failed or could not be completed with a good result. There could be numerous other things that can interfere with a test.
Toyota P0502 OBD DLC Error Code Vehicle Speed Sensor Circuit Low Input