Basically, the garage is waiting for the OBD I/M readiness monitor to be set. When you clear the ECM, there are stored parameters that also get erased, and the car has to "relearn" those parameters. Some of those are learned very quickly, others - like the EVAP codes, can take a while to reset. The I/M readiness monitor - has to complete so many "drive cycles" before it enters a ready mode, before it hits those drive cycles, your emissions testing can not be done. There is no easy formula that you can follow to set the I/M readiness - though some have followed a universal guideline and it worked for them.
Example of that guideline is as follows:
- Start the engine, idle the engine in gear (automatic) or turn on accessories (manual) for 180 seconds
- Accelerate to 55 mph at 50% throttle
- Hold steady speed of 55 mph for 180 seconds
- Coast to 20 mph (no braking or depressing the clutch)
- Accelerate back to 55 to 60 mph at 75% throttle
- Hold steady speed of 55 to 60 mph for 300 seconds
- Coast down to a stop (without braking)
- Shutdown engine, wait 240 seconds, restart and repeat above
As you can - nearly impossible to accomplish those setups unless the car is on a dynometer, might be able to pull that off in more rural areas, but not in the city. Driving normally should hit all those conditions over the course of a few hundred miles, several days. The caveat is the speed, if you drive almost exclusively in the city - you may not hit the high speed threshold for setting the I/M readiness monitor. The flip side is driving almost exclusively highway - you may not hit the start/stop engine cycling, low speed, coast down threshold for setting the I/M readiness monitor. The trick is driving a few hundred miles of mixed city and highway with some periods of idling, startup/shutdown, varying the speed/engine load over a wide temperature range.
Sort of a PITA, as the actual emissions are probably quite clean - most states that do emissions testing only query the ECM's I/M readiness monitor. The car could be billowing smoke and raw fuel, but if the ECM's self test checks out OK - you pass. Some places run an I/M 240 emissions test - which involves querying the computer and running the car on a "treadmill" or dynometer over a prescribed course.