My car was idling low and almost stalling, so I took the advice of some others and messed with the plastic screw that adjusts the idle speed. It worked well, and the car idles right, but now my check engine light came on. What did I do?

 

check your O2 sensor, and MASS airflow.

 

The Check Engine Light is evidence that something is wrong. That
"something" may be any number of things, some major and some minor. It
might be nothing more than a loose fuel filler cap. The Engine Control
Module (the computer) detected a problem and turned on the Check Engine
Light. At the same time it stored one or more Diagnostic Trouble Codes.
Get someone to do a DTC readout for you. This is not a big deal, does not
require specialized "dealer only" test equipment, and should take less than
an hour. When you know the stored DTCs, you will have a fair idea of what
repairs are needed.

If you like to do your own "wrenching" you can do the DTC readout for
yourself. Go to your public library and borrow a Chilton's or Haynes
manual for your vehicle. It contains detailed instructions and a table
which "translates" the numerical codes into words. No special tools are
required. You will also find instructions to do a DTC readout and reset
the Check Engine light at Steve Cutchen's excellent MaxFAQs page:
http://web2.airmail.net/scutchen/max_faq/

It's possible that the Check Engine Light did not turn on because of the adjustment of the idle speed. Perhaps the low idle and stalling were symptoms of an oncoming problem, and your adjustment masked the problem but did not solve it. The time has come for a proper diagnosis and remedy.

 

Chances are the poor idling was caused by something mechanical (spark plugs,clogged/dirty injector,dirty air filter) rather than a plastic set screw that probably did not move by itself or require adjusting to begin with. I would pull the engine code just to see what it gives and then put the screw back where it was (if possible) and go from there.

 

I checked my code with the diagnostic test, and I got a code of 0201, which says IGN SIGNAL-PRIMARY, what does this mean, it doesn't sound good.I guess I might call the Nissan dealer.?

 

This information comes from the '99 Maxima factory service manual. I believe it also applies to your '96 Maxima.

Diagnostic Trouble Code 0201 indicates that the ignition signal in the primary circuit is not entered during engine cranking or running. The possible causes are ...
- Harness or connectors (the ignition primary circuit is open or shorted.)
- Power transistor built into the ignition coil.
- Condenser
- Crankshaft position sensor (REF)
- Crankshaft position sensor (REF) circuit

I would have expected to see DTC 0201 along with other DTCs. Were there any others? A misfire on any individual cylinder should set a code, and a problem with the CPS should also set a code.

Is your battery fully charged? A low voltage condition resulting from a discharged battery can cause problems for the Engine Control Unit (the computer).

Is there some vehicle history which may be relevant? Has the car ever been in a crash? Has some seemingly unrelated engine work been done recently?

 

I went to the nissan dealer, and had my car checked out. There was no problem. The codes that they got were throttle position sensor, and the mass airflow sensor, he said both of which can be included in code 0201. When I adjusted my idle speed,I unplugged the wire harness and it set those off for some reason, but they checked it out and everything is fine. Thank goodness because a mass airflow sensor is $550! Thanks for your help.