Hello, I have a 2000 SSEi with 59,340 miles. Tonight when I was driving home the service engine soon light came on. The engine was running smooth, no misses at crusing speed and none when I accelerated up to 80 mph. I also drove through some stop and go traffic and the car performed just as it always has.
When I got home I checked the coolent overflow tank and it was at the proper level and the oil dipstick looked good, there was no leaks on the engine or on the parking lot. The exhaust smelled normal and there was no funny smells coming into the cabin or from the engine compartment.

Then only thing that I noticed a little different on the gauges was the temp gauge was operating about 1-1.5 marks lower than normal at crusing speed, and when stopped at a light the temp would go back up to normal.

Does anyone have any ideas? Thanks for the help.

your gonna need to get the codes scanned.....Autozone or Advance will do this

make sure that you get the exact code and report it back here

Ignoring the SES light is not recommended. Each person can make that choice for themselves, But I wouldn't recommend that you wait and see. It may be something as simple as an evap code, but let'* find out for sure. All it takes is a few moments of your time.

Check out the techinfo article here:
http://www.bonnevilleclub.com/forum/...e=article&k=74

it could be 50 things that could cause the light or more...many of them do not affect performance or driveablity

New cars are loaded with sensors to let you know before something fails, so if you ingore the light you could have a more expensive problem on your hands later on.

Brender.. as mentioned, please have the code read for free and report the code back. We can help you decide at that point if something should be done.

You said 1-1.5 marks. What mark is the engine termperature reaching? If you do not reach a certain temperature the car will set a check engine light. Also that would indicate a faullty thermostat. Which is easily changed out. If not done, your car will run cold, your MPG will suffer and you may notice other undesireable things.

If this is the code that you find, we'll be more than happy to walk you through a thermostat replacement.

Had you just started up from a stop where the car was shut off and the engine had not completely warmed up when shut down and restarted? That'* when the fuel system'* ability to hold pressure/vacuum is checked. Did you check your gas cap seal and the tank neck for crud keeping it from sealing?

I stated the car and let it warm up for about 10 minutes and the light just came on when I was going down the rd at 59 mph.

I just checked the gas cap and it seemed like it was sealed up properly.

I did notice this morning coming to work that the instant miles per gallon was anywhere between 3-5 miles per gallon less than it normally is at 60 mph. Normal is 28-30, today it was consistantly 24-27.

The temp gauge is normally one mark below the 200* symbol.

I am off to autozone to get the code read. When I get it I will post it here.

Thanks for all your help.

Just got back from advance auto. The code is P0128, under the code it reads, Coolent temp., below thermostat, regulating temp.

That would usually indicate that your thermostat is stuck open and not allowing the car to get up to temperature. Before we send you off for a thermostat and gasket, please check the overflow bottle level and if the car is cool, check the level in the radiator itself.

Always be careful on a warm engine of the coolant in the radiator, it can be hot and burn.

Once you have verified the coolant level is up to par, then it'* time to change the thermostat.