93 SC2 Trouble code questoin

Hello all, my "check engine light" has gone on a few times in the last
couple of weeks, usually just after a hyw run and when I am stopped waiting
at lights just after that. I looked up how to jump the pins to read off the
flashes to see why and it returned the code 12; checking this code it says
"Diagnostic Check Only"; can anyone let me know what this actually means?

I do think I may have a temp sensor problem, as I recall these engines have
2 temp sensors, my gauge is fine but in the summer the fan would never turn
on, during this winter the car starts fine but wants to stall when ever I
stop (engine cold or hot).

Thanks for any help
Brent

 

Code 12 means that internal diagnostics are active and self diagnostics is
OK.

As for the temp sensors. One sends signal to gauge. The other sends signal
to PCM and this controls both rad cooling fan and adjusts fuel injector
pulse width based on engine coolant temp. Thusly when engine is cold, more
fuel is added. Typical problems when control temp sensor goes are cooling
fan always or never runs and car is hard to start in cold weather. the
colder, the harder.
There is also an Air temp sensor in the intake air duct - not used for
cooling fans, only for fuel control.

Both temp sensors are by the EGR valve. The upper one is the Gauge sender -
only one wire to the connector. The lower one is the Control PCM sensor - 2
wires in connector. Make sure they are not mixed up. There have been several
reports here (including mine) of the control sensor failing. Fairly easy
and cheap to replace yourself.
You may have a secondary or another problem causing the stalling when you
stop. This is more typically related to IAC (Idle Air control) They can get
dirty and/or stick and cause problems with idle control sometimes causing a
low RPM drop bounce when returning to idle. Dirty pintle pin and/or port
does not allow sufficient air when in full closed idle position. It can be
removed and the pintle pin cleaned and the port cleaned out - sometimes will
work normally after that if the unit still operates correctly. It would need
a new o-ring seal when re-installing.

 

Thank you Mike,

That information was excellent and very helpful, I didn't know about the air
temp sensor in the air intake, this problem as you describe it for the IAC
sounds like what is happening to me, the stalling only occurs when I let off
the gas coming to a stop, the idle drops off to 1-200 RPM, sometimes it is
OK and the idle comes up, other times it stalls, once restarted it sits at
idle fine it's just when letting off the gas.. This also only happens when
the outside air temp is cold..the colder the more it seems to be a problem..
but I don't really have a problem starting the car in the morning, I'm just
east of TO and the last few weeks we've seen -25 with wind chill of -35, the
car was hard to turn over, even had to boost it one day to turn over but
once the starter spins OK the car starts fine and holds idle fine while
warming up.

Thank you again
Brent