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Computer operated engines are pretty straight forward, people tend to over complicate things when there is a problem.
First, the computer runs a very basic "if/then" software, it doesn't do any "thinking".
1."if" VSS(speed sensor) is 0/"then" check TPS voltage(throttle position)
2."if" TPS voltage is less than 1 volt/"then" check crank position sensor
3."if" crank position sensor(RPM) is higher than 750/"then" close IAC valve
4."if crank position sensor is lower than 750/"then open IAC valve, (loop back to 3.)
The computer relies on sensors to be accurate, and runs basic connection tests but has no way to know if data coming in is correct.
If a sensor doesn't respond or responds with voltage that is out of range then computer will set a code for that, some codes will cause the CEL(check engine light) to come on, some won't if a retest shows OK.
The computer also tests "controls" like the IAC valve, when engine is first started the computer will open the IAC valve all the way, raising RPMs to 1,800+, then it will lower RPMs to pre-set cold or warm idle setting, if IAC doesn't respond this way it will set a code.
The computer stores all these codes in memory whether the CEL comes on or not.
So it is always a good idea to read these "history" codes and then clear them.
The history code may show a problem that is self correcting but is the first sign of a failure coming.
Clearing history and then rereading the codes in a week can tell you more specific areas to look at.
Never swap out sensors or controls, hoping to fix a problem.
TPS is a sensor, it is easy to test with a volt or OHM meter
IAC valve is a control, also easy to test with a volt or OHM meter
Almost every sensor and control can be tested with volt or OHM meter
So do you have a CEL showing while driving?
Does the CEL come on when key is first turned on and then go off?
So bulb is good
Can you stop by an auto parts store and get the codes read for free and then cleared?
I would try the neutral thing, just to see.
Last edited by RonR; 11-03-2013 at 11:34 AM.