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On Board Diagnostics Code 900 1991

Those trouble codes basically tell you that at some point in time the ECU could not achieve an acceptable fuel mix -typically because of an improper air or fuel supply (unmetered intake air, incorrect fuel pressure, dirty fuel injectors) or faulty engine management sensors (AMM or O2 sensor).

Before you get too worried, always clear the trouble codes and wait for them to re-appear. As you've been having rough running problems that are now cured, the problem may simply have been transitory. An engine will often set these kind of codes if it stalls or dips wildly below normal idle rpm and the TPS switch hasn't been tripped (as in your case).

Otherwise Greg has put you on the right path. In addition to the possibilities he mentioned (and the IAC valve is indeed an unlikely contributor) you should look carefully for intake air leaks -split/disconnected vacuum hoses (check both ends), idle air bypass hoses, main air supply tube (!!), leaks at the throttle body gasket. Perform a basic test on the fuel pressure regulator as per the FAQ. Before getting too suspicious of a faulty AMM, try thoroughly cleaning the AMM electrical connector with spray contact cleaner and re-packing the female contacts with dielectric grease -do the injector, TPS switch and system relay (radio noise suppression relay) connectors at the same time.
--
Dave -not to be confused with a real expert






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