How do you clear codes for emission test?

Answer 1

You can generally clear codes in one of two ways:

1) Disconnecting the battery for a certain amount of time (30 minutes to be safe). This is NOT recommended as it can cause all sorts of problems with your car's electrical components and its computer.

2) Using a code reader scan tool (like the kind they lend free you at most major auto parts store) to read the code and then clear it. When you plug the tool into your vehicle and it reads the code(s), it will give you the option to clear them.

HOWEVER, Neither of these will allow you to pass an emissions test. Most states now rely on your car's OBDI II computer to tell them about your emissions system. If an emmissions related code (ex P0420) has been manually cleared, the computer will show that and you will NOT pass the test.

Oftentimes, a bad emission code can be caused by a faulty oxygen sensor(s) and your actual emissions are still within the legal limits. Unfortunately, it will not matter if, like most states, your state stipulates an OBDI II test. If you live in a state that allows the old fashioned "sniffer" test (actual measuring of the emissions), this may be your best bet to pass. If you do need to get your emissions system worked on, make sure you have the oxygen sensors tested for proper performance before you agree to replace your catalytic converter(s).