Why doesn't it start?

Remove the oil filler cap on valve cover and have someone turn over engine. If valves are moving the timing belt is OK. , if not the timing belt broke or is loose and may need replaced. Check internal timing pulleys as well for damage.
Also, check spark plug wires and replace as needed.

what is the make and model and year please.? our mechanic may not have set the timing right. it can be a teeth off. I need the make and model

sounds like timing belt or chain has broke or maybe a timing gear. I would not advise trying to start it if this is the problem because you run the risk of bending the valves. TRY this to determine if it is the timing. Take cap off off valve pan cover " where you put the oil in " and using a flash light look down into the hole and have someone try to start it. IN most cases if the timing is out wou will see no movement of the rocker arms....hope this helps and good luck....

This make and model doesn't have a fuel pump cut off switch, the switch is only activated by an impact of 3 G's or more on cars and trucks that do use the switch. Spray starting fluid into the air intake of the engine, if it doesn't start you do not have a fuel pressure problem. Lots of things can keep a car engine from starting, you never mention any tests for fuel pressure or spark or proper valve timing (cam drive belt broken or jumped)

battery cables could be corroded or worn out.bad battery cables will cause starting problems. if starter okay. if engine turning over very slow.try turning engine with a large 1/2 inch socket and 1/2 socket wrench.if engine hard to turn could be locking up.if engine turn over easy with wrench. ignition timing and valve timing could be off due to worn timing chain.if engine turning over normal speed when you try starting it.could have intake leak.egr valve could be stuck open.could have faulty crankshaft position sensor or faulty camshaft position sensor.vechicle could be in theft security mode.best to have vechicle code scan that will save you time and money.

I hate to give bad news, I have forgotten when Chevy started putting Coil packs on that particular vehicle. If it has a Distributor pull the distributor cap and crank the motor over while you are watching the rotor assembly. If it doesn't turn the Timing Chain Broke. If it has a coil pack, pull one of the Valve Covers and crank the motor while watching the valves. If the valves don't cycle the the Timing chain is broken. Now, assuming the timing chain hasn't broken then you need to check the fuel pump.

This is a common problem. The marks you are talking about actually line up several times during the cycle of the motor. You need to turn the motor by hand until the number 1 piston is all the way up and the next piston in the firing order is starting it's upward movement, this is Number 1 Top Dead Center. Look at the valves and make sure that both valves on the number one are closed and only the intake valve is open in the next cylinder to fire. That should set the valves to Number 1 TDC. Then align the timing marks and replace the timing belt or chain. This should but you at 0 degrees timed and allow you to be able to adjust the timing as needed to get the motor running smoothly. Good Luck.

I would start by checking the bleeder valve on the fuel rail to make sure its getting gas and has good pressure.

If you have spark and fuel and compression and the timing is correct like you said, it will run. One or more of these things is not correct. Proper diagnosis will find out which one(s). The ignition timing is not adjustable on the 2003 2.3L engine, so I sure don't know what you reset???

The place to start would be to scan your Engine Control Module (ECM) for fault codes.

Please also review this article:

What Else Could Be Wrong?

Could be dirty fuel injectors. solution: pour fuel additive such as SeaFoam or Lucas injector cleaner in the gas and drive.

Ignition timing could be off. Check timing.

OR, could be throttle position sensor is going bad. If cleaning the injectors doesn't fix it, then the sensor is going out and needs to be replaced.