'service engine soon' light comes on one day, then is off the next--no pattern-- no intake or liquid leaks found by mechanics?? Also, 'low coolant' light comes on now and then, no leaks found; not even the water pump. ??

See my answer to your related/same question which I include again for your reference:
You must immediately check your engine oil level and take appropriate action: unless you have a defective oil pressure sensor, oil pressure light usually indicates that your engine oil is not flowing properly. Some possible issues: low oil level (leaking engine?), blocked oil filter (when did you last get your car service) or defective or blocked oil pump due to excessive dirty engine oil.
Also check under your car to see if you have any oil leaks: this usually indicate that you have defective engine gaskets or oil seals causing your oil (pressure) warning light to come on.
Hope it is of assistance. But I would consult a motor mechanic asap as you engine could soon fail due to the lack adequate oil lubrication.

Need a different mechanic

They never tested anything,
if you got codes

for the engine light to come on there will be codes stored
It is that the mechanic is not using the correct scanner with the right protocol installed to read them

I have a 1999 Mercury tracer. I was driving down the highway two weeks ago and my check engine light came on. i went to autozone and he said something about misfire on bank and vacuum leak. He said it bascially was a tune up. i got in my car the next day and the light went off. it didnt come back on. Last sunday i got a tune up and a oil change. i was driving to work today when my service engine soon light came back on. could it be because its been warm here in st louis and all of a sudden its cold. or do i just need to get it reset. i hope its nothing major because im broke right now and cant afford to get it fixed

code p 0496 refers to evaporative emission (EVAP) system---high purge flow===causes ---wiring --EVAP valve---mechanical fault

code p 0171system too lean bank 1===causes --intake / exhaust leak---AIr system---MAF/VAF sensor--fuel pressure too low/---pump--injectors ---Heated oxygen sensor (HO2S)

with the first code it could be a faulty vent valve or solenoid
with the second code , it is probably a faulty HO2S unit ( the one in the exhaust manifold before the cat converter)

code p 442 refers to evaporative emission (EVAP) system---small leak detected
it also refers to large leak detected --fuel cap off/loose === causes hose/connection,-- intake leak--EVAP canister---EVAP canister purge valve --mechanical fault --- EVAP pressure sensor

Your vehicle uses GM Dex-Cool from the factory which is a coolant that is red in color. You more than likely have a leak so the system is low and therefore the coolant light is acting up.

There are several Technical Service Bulletins on this year Saturn for intake manifold gasket leaks for one, but it could be from a lot of places.

Washing the engine and letting it dry, then running it the next day and hunting around with a flashlight may help pinpoint the problem area. Check hose clamp areas of course.

For difficult leaks they make a Dex-Cool compatible UV Dye that can be seen with a UV light at a repair shop.

best wishes!

its probably a head gasket, get your wallet ready

If you get ck engine light on , you have a cyl. number
that misfires , ck vacuum leaks on intake manifold around that area aspecially intake manifold gasket.

get a new mechanic, need to check for vac. leaks, poss needs tune up. alot O2 codes i have gotten have been created by vacuum leak or warn out plugs, rarely have i had to replace a O2 sensor.