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Was the light on or did the tech say the system was not ready? They can be two different things.
A light would need to get scanned or a code reader attached to the car may do it. Most auto part stores will hook up a code reader for free and let you know what codes are present.
If the light was not on but it stated system not ready on the smog machine that can be due to the battery being disconnected. Its basically saying that the computer needs to go through drive cycles so it can test the its systems.
Before you start, make sure your negative battery cable is disconnected. Once the new computer is installed, reconnect the battery cable. Since the "new" computer isn't connected to a power source, everything in its memory should already be reset, and it will be ready to go.
if you didn't have a check engine light on before you unplugged the battery, you'll be all set once you drive the truck for a couple more days. the engine control module on 1996 and newer OBD2 system vehicles run emissions system checks called readiness monitors. each system check requires a certain "drive cycle" or driving condition and situation. for example, your heated O2 sensor check requires you let the vehicle run at idle from cold startup for a certain amount of time. somewhere around 5 minutes. so i suggest you give it 3 days or more to be safe before you go back for a re-inspection. better yet, go to a local parts store and ask to have them check your readiness monitors. make sure they don't clear the monitors or any check engine codes!!!! you'd have to start all over again.
has the battery been disconnected, drained, or computer codes been cleared? all of these will erase readiness monitors in the computer and will have to be driven to reset it's hard to say how far because it varies on different vehicles
Is the engine cold? It will say that if the Oxygen sensors aren't warm yet. Try driving it for 10-15 miles before you have it inspected. If that doesn't work the heating elements might be bad in the sensor and you will have to replace the Oxygen sensors