Installed low mile engine in 2006 Toyota avalon

I think maybe either your mechanic is a cobbler or you did not understand what you were told. Some of the emission monitor systems may not be ready and driving up to 100 miles may be necessary, but if the check engine light is illuminated then you have issues that need to be repaired.

When the battery was disconnected (as it should be during any major work like this) or someone resets the OBDII codes with one of those cheap code readers, the I/M (Inspection and Maintenance) OBDII monitors are all turned off. Each has to be running in order for the actual emissions / performance tests to run. Until all* the tests can run, the vehicle will fail to pass inspection in states that require testing. The 100 mile figure was just pulled from thin air. Each I/M has a specific driving cycle that must be completed before it will show as running. Real mechanics know how to complete these cycles quickly. The alternative is to let the owner drive around for a couple of days and re-test to see if all the I/M monitors are running. I recently got a car out of storage and 3 of the 10 I/M monitors were not running. I drove over to Walmart and out to Mom's (a combination of urban and highway driving) and when I got home all 10 monitors were running and good to go. Total miles: 19.5. _________________ * Some states -- like New York -- will allow the vehicle to pass if one I/M monitor is not running.