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GMTechBernard, Shop Foreman - GM World Class Tech
Category: Chevy
Satisfied Customers: 2158
Experience:  GM
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2002 Chevrolet Blazer: Check engine soon light is on..emissions

Customer Question

Car is 2002 Chevrolet Blazer. Check engine soon light is on. Took for emissions, failed due to light being on. Replaced the OB part. Took back to emissions, still failed, they stated that the car needed to be driven for approx 200-500 miles before the fix would register. Why does replacing the part, make it pass emssions?
Submitted: 7 years ago.
Category: Chevy
Expert:  GMTechBernard replied 7 years ago.
Hello! When a check engine light is diagnosed and a part is replaced, it is common to then clear the code so that the light goes out. Clearing codes, also resets the "readiness monitors". A readiness monitor just states wether or not the on-board diagnostics have run. For example, you have an EGR monitor, Catalytic converter monitor, Oxygen sensor monitor, etc.... Each monitor has a series of tests before it will switch to "ready". On a 2002, you can only have one monitor "not ready", (at least here in NC, but other states are similar). In order for all the tests for a given monitor to run, it needs to be driven, however the statement of 200-500 miles is not correct, because you could fill the tank up and drive for 500 miles and not a single monitor would set. It needs miles, but it has to be broken up into "drive cycles". A drive cycle starts from a COLD start (sit overnight) and the fuel tank must be between 25% and 75% for certain tests to run. You also need to acheive various speeds (in town driving and highway driving) as well as several long decels (get up to speed and then coast for awhile). Your best bet is to drive it like you normally do for a week (that would allow for multiple drive cycles) before returning to get tested.