My 2001 pontiac gunfire will not pass emissions. They said they can't read the computer and it needs to be reset. How do I do that?

check the fuse for the obd port --check the connections and wiring for thew port

What codes were reading for the engine light being on ?

A new cat what was the original code. Usually you need to drive the car for a while after changing a battery (1/2 hour to an hour) on the road so that the cpu as all the data it needs before going for etest especially the new etest machine. The place that change the cat is it the same place that is doing the emission test?

The only way your car will pass the emission tests is to get the car repaired, period.

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Sometimes the emission light will come on if the gasket on the gas cap is worn or dirty, thus not sealing properly. If it doesn't look worn, then try cleaning it.

Has the battery been disconnected, or has it gone dead? Every time the battery is disconnected, all trouble codes are cleared, the check engine light is turned off, and all system monitors are reset. For the monitors to get back to a 'ready' status, the vehicle needs to go through one or more (sometimes quite a few) drive cycles.
When you go to the emission station, they check for codes, AND also check to see how many of your monitors are in a 'ready' status. (This keeps people from disconnecting the battery, and thus clearing any trouble codes, in order to pass emission tests).
If you have not disconnected your battery for a while, but the monitors are still not showing 'ready', you need to have a qualified shop check out your computer and other electronic systems.

http://shop.ebay.com/i.html?_kw=2001+Honda+Civic&_kw=engine&_kw=computer
This is a link to an ebay page where you should be able to pick out a new computer pretty cheep for your car hope it helps.

You need to have the the computer reset.

You might have to go to a Cadillac dealership to get it reset. Some sections of the computer are locked out so only the manufacturer can fix it.