2006 Saturn Ion gas mileages

The Engine Computer will set a diagnostic fault code and turn on the Check Engine light to tell you if something is causing excessive fuel consumption or is potentially causing too much emissions. Beyond that, if you have a scanner that displays live data, you can look at the fuel trim numbers. If they're high positive, the computer is adding fuel above the pre-programmed amounts, and you need to determine why. If the numbers are high negative, the computer is subtracting fuel and you need to figure out why too much is going in. A leaking injector and high fuel pressure are likely suspects.

If the fuel trim numbers are normal, as in positive or negative a few percent, look for a dragging brake or low tire pressure. Those will cause high fuel consumption while the fuel / air mixture is okay.

We need exact fault code numbers to know how to start the diagnosis, but coolant temperature is one of the inputs the computer uses in its fuel metering calculations. Liquid gasoline doesn't burn. It has to be a vapor or it will go out the tailpipe, wasted. When the engine is cold, it's harder to get the fuel to turn into a vapor so the computer will command more fuel in hopes a high enough percentage will vaporize to make the engine run right. (That's what chokes do on engines with carburetors). A warmed-up engine needs less fuel, but if the thermostat is preventing the coolant from getting hot enough, the computer is going to add more fuel than is needed.

P0128 - ECT Below Thermostat Regulating Temperature

That means the thermostat is leaking or opening too early and not allowing the coolant to get up to 195 degrees. Any engine with a carburetor needs a choke to dump in way too much fuel in hopes a high enough percentage will vaporize by the time it gets into the cylinder to burn properly and produce sufficient power. Once the intake manifold and cylinder walls are warm, the fuel vaporizes better so the choke is no longer needed.

The same thing is true with fuel injection. The cold engine still needs too much fuel because only a small percentage of it vaporizes, but now that extra fuel is programmed into the computer instead of relying on a crude mechanical choke. The computer looks at coolant temperature and intake air temperature to determine how much extra fuel is needed. With the fault code you listed, the computer thinks the engine is still cold so it will likely be adding extra fuel. You can verify this with a scanner that displays live data. It will list the coolant temperature. Most commonly when the thermostat is opening too early, you can expect to see coolant temperature of around 140 - 160 degrees. Normal on GM products is between 195 to 215 degrees.

There's lots of good ones out there but they're expensive. You might ask at the auto parts stores that borrow or rent tools, but usually they just have code readers. Those don't display live data.

I use Chrysler's DRB3 because with an extra plug-in card it will work on their older cars, and it will do emissions-related stuff on any brand of car sold in the U.S. 1996 and newer. A lot of independent mechanics bought them too for that reason. If you do a search on eBay, you'll find hundreds of scanners for sale. Snapon makes a bunch of models but they are really proud of them and charge accordingly. Genesis makes some popular models. The GM scanner used to be the Tech2. I suspect they've gone to something newer.

For what you need this will probably work fine. I recently bought something similar for a friend, and while it does display live data, it's rather irritating in that it takes readings, then displays them for a few seconds while it gathers the next readings. A lot can happen that you won't get to see while the display appears to be locked up for those few seconds. There's a reason these are so inexpensive, but they're most likely still a good value.

I don't know what is normal fuel pressure for your car, but most today run around 50 pounds. If yours is high, the Engine Computer will detect a rich condition and set the appropriate fault code, or you'd see high negative numbers for fuel trim values indicating the computer is subtracting from the normal fuel metering calculations in an attempt to correct the mixture.

The first three cars you listed are OBD2, (on-board diagnostics, version 2) used on all 1996 and newer cars sold in the U.S. Your newer car almost certainly uses the even newer "CAN BUSS" system. That's a totally different computer language. You'll need a scanner that communicates in that language. Welcome to the world of repair shops. They spend thousands of dollars every year to buy new equipment and updates.

I don't have a service manual for your car. Most of the time the thermostat sits under a housing at the end of the upper radiator hose.

Your thermostat is a bit unusual in that it uses a rubber o-ring type of seal around the thermostat. That does a pretty good job of sealing by itself, and you shouldn't need anything else. On engines that use a thick paper gasket, it doesn't hurt to add a light film of gasket sealer to both sides. That will fill in little scratches or pock marks from corrosion, and it will make the gasket easier to clean off next time.