The pump should start up for about a minute after a old start. It pumps air into the exhaust flow during this time to speed up the start of chemical reaction in the cat. Does the pump run? When running there is a vacume operated valve which opens up to allow the air flow. The valve and pump motor are located on the exhaust side of the engine. You should have a vacume at the vacume line.
The vacume is controlled by a solonoid on the intake side of the engine, you will see the vacume line running over the top of the engine. Make sure it is intact and that the rubber connectors at the ends are not leaking. This is the most likely cause.
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David Hunter