Check engine light on, Replaced Gas cap, and Air filter. Engine runs good, ran 300 mi since cap and filter change.

The OHV engine requires periodic valve adjustment. Typically every 300 hrs.

Another possible cause is an ignition module failing at operating temperature. Run until it fails and check for spark at BOTH plugs.

Wich o2 sensor did you replace? If it's the upstream one you may have a clogged Cat converter, this makes the engine run badly due to restricted exhaust flow. When you first turn the ignition to the on position make sure the "check engine " light comes on momentarily, if you notice it come on the the above is likely your problem. Scan the car for codes anyway (you still might have codes in memory!)

Best of luck!

Did you check for spark? It sounds like you're not getting spark. Check for spark. If you are not getting spark then it sounds like you might have to replace the coil packs. Is it running a L4 engine? If its running a standard chevy L4 its probably the coil packs. Try to replace them check again for spark.

Should have drained the tank. Not only was it old gas but it may have had some condensation in it.
Now that you have a Check Engine light, hook up a code reader and see what code(s) are there. Then you can get back here and I'll see what I can do to lead you in the right direction.

signs of a bad fuel pressure regulator. the vehicle runs rough, the engine will not start usually in the mornings. many times engine starts but stalls shortly after starting. a bad regulator can result in fouled plugs. black soot on plugs, blake smoke from tailpipe, running rough, stalling. you could have residue in fuel tank and need to change fuel filter again. have a very good day !!

Sounds like a computer sensor which can only be determined by an auto mechanic with the proper tool.

i have 2003 trailblazer ltz 153 000 mileage i have code po440 .i changed gas cap and turned off the chech engine light... i made 300 miles and it was good and today morning i drove a mile and light turned on ..... what do i have to fix ...thanks

the check engine light is not caused by any wrong fluid level or dirty air filter or anything else like that, it is caused by well over 100 faults that can occur in the computer controlled electronic engine and emission controls, u will need to go to a Kragen or Autozone parts store and get a free OBD 2 fault code test to get the code that set your light.

The check engine light has nothing to do with the oil, water etc. it signals that a fault has occurred in the engines electronic controls and it has failed a test of some system that the cars computer runs every few minutes while the engine is running, it can be one of a 100 different systems that made the light come on, take the car to a Kragen Auto parts store or another chain parts store like it and ask them to run a OBD 2 system test (it's free), this will retrieve the trouble code(s), post the code(s) back on this thread and I will explain what it will take to repair.

try leaving gas cap loose on your next fill up and see if that helps. gas tanks need air and if gas cap vent is not letting air in the truck will acted like it is running out of gas