My '99 2.4 Malibu started having some hesitation when running at lower speeds
and gradually over the next month progressed to stalling out at lights, with
hesitation from a start and at low speeds much worse. I replaced the plugs,
fuel filter, fuel pump, and air cleaner. I then tried a new IAC valve to no
avail. There was no check engine light until a few miles after I replaced the
IAC valve, then it went on. I went directly to a shop and they extracted 2
error codes - P0172 and P1171 with freeze frame data. Wierdly, the check
engine light was on after I left the shop, then went off after 5 miles.

What would be the probable causes of this problem?

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