I have a 2001 VW golf that I just bought. It passed smog checks no problem. But now the Engine Light is "ON" and the guy at the shop told me that the code is 420. What is code 420?

run the fault codes to ensure that the EGR valve is faulty. Engine lights to not always come on unless there is something drastic going on with the engine /gearbox so that you can stop the car before damage is done.

Codes and the CEL can be erased with the same scanner used to retrieve the codes. It is a separate function of scanners.

Find someone with "VAGcom" or VCDS cable and software.

ALWAYS PERFORM TESTS AND REPAIRS ON THE FIRST DTC AND RETEST AFTER EACH STEP.ONE SENSOR OR COMPONENT FAILURE CAN CAUSE OTHERS TO FAIL AS A RESULT.MEANING THE FIRST CODE COULD HAVE CAUSED THE OTHER CODES TO CYCLE.SO ALWAYS DIAG THE FIRST CODE STORED AND EACH ONE AFTER AS THEY WERE READ DURING THE SCAN

There should be a 10amp fuse in the fuse box for the instrument panel that powers the light. The light is grounded by the computer.
Have you checked the fuses in the passenger compartment ?

Whenthe check engine light comes on, one or more diagnostic trouble codes (DTC) are stored inthe engine control module.
super vag k can plus can help you clear the fault codes.

Thanks for choosing FixYa and welcome. When the light is on, you can test the computer and find out what is wrong with your vehicle. As soon as it comes on, take it to Autozone to have it scanned. It is free there and then you can tell you shop guy the code you have got.

the glow plug relay is located onthe pass side valve cover ! it has two large cables on it and two small ones

take the dash assembly off(about 4 screws) and take out the light bulb that illuminates that "check engine" sign.

Have somone with an OBD II reader plug in and check for codes.