We just replaced the knock sensor on our 98 Legacy, after failing an AZ emissions test. Our Subaru certified mechanic told us that, before returning for retesting, we needed to drive the Legacy for 25-30 miles, so that the OBD computer would reset.

 

We drove 150 miles, but when we returned to the emissions testing site, we were informed that the computer had not yet reset. Our mechanic now advises us to put on some highway miles at 60+mph.

 

We don't want to return to the emissions center without some confidence that our car is ready for inspection. Any advice on how to drive the car so that the computer is reset?

 

Thanks in advance.

Unhook the negative battery terminal for at least one minute, then reconnect. This should reset the computer.

It has to do with something called readiness monitors, which are present on 96 and newer model cars with OBDII. Your car runs test on various sensors and systems in the car as the car is driven, and when it completes those tests, the computer marks those systems as "ready". Some of those tests take time to run...hence the advice that you put a certain amount of mileage on your car before you attempt a retest. Unfortunatly, the only way to tell if a monitor has been set to ready that I know of is to hook the car up to a scantool. They will do this for free at autozone. Unhooking the battery cable or resetting the ECU with a scantool will result in the computers memory being erased, and thus the car will have to run through the tests again to set the monitors to ready. I know in Texas at least, my car model (a 96 legacy 2.2L) is flagged for having trouble setting it's readiness monitors, and when they run the emissions test, the technician just does a two speed idle test if the monitors come up as not ready. I don't know if that is an issue with your car though. Best of luck...

Ha! I can be useful

Take a look here... you'll need Adobe Acrobat to read it. Take a look at the end of it, where they list a set of generic instructions to reset the drive cycle. Hopefully they're not too stringent, 'cause they look to be a pain in the arse.

 

I'm not sure about any other states, but NY won't straight up pass you if those tests aren't completed... there's a few loopholes if I 'member right, but it gets complicated.

My bad. My earlier suggestion will actually make your problem worse! Listen to the others. They are correct.

 

I'm sorry - I am used to OBD I.

Thanks for all the replies!!