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Q: Engine revs up and down when in park and sometimes in drive/reverse; have replaced IAC valve, and the map sensor 1998 Honda Prelude

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Semiautomatic; friend drove car in 1st gear going 40+ mph, revs up and down when in park, and some/most of the time while in drive or reverse. We have replaced the IAC valve and the MAP sensor, when running diagnostics reads P0505 any ideas?

My car has 283000 miles.
My car has an automatic transmission.

Hi there - the P0505 code you are getting indicates a problem with the idle air control circuitry. Since you have replaced the IAC valve, this could still be a problem with the connectors or wiring to the IAC. There is a specific "re-learn" process to be performed after installing a new IAC as well - perhaps this was not done, or done correctly? There are also other items that can lead to rough idling: dirty or malfunctioning throttle body or throttle position switch, or air leaks in the intake. I recommend a rough idle inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.

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