Hyundai Elantra, 2007, GLS, 2.0L, Automatic, 99k, Northeast/New England



So I'm trying to fix my mother's 2007 Elantra GLS. Had trouble starting a few days ago and isn't starting at all right now. The engine is turning but it's not starting. It also broke down on the highway after not accelerating past 60mph, struggling on a hill, and not upshifting at 5,000RPM. got slower and slower until the engine cut off. This happened and now it isn't starting.

Some Background info:
recent oil change.
all the fluids are fine. gas, oil, water, coolant, all full.

Alternator recently replaced.

Hyundai dealership recently updated the computer as a recall.

I changed the spark plugs after it wasn't starting.
I also replaced the o2 sensor and the crankshaft sensor(they both showed up on an OBD test)
I cleaned the Mass airflow sensor
I checked the Fuel pump fuse and it's fine.
I also checked the fuel pump relay, it's fine.
The car battery is also fully charged.

The o2 sensor and crankshaft sensor are not showing up on the OBD test anymore.

We removed the rear seats and accessed the fuel tank panel, and can hear it buzz when the ignition is set to "on".

Trying to get my hands on a fuel pressure tester to see if the fuel pump is working at all. I'm thinking it could either be a clogged fuel filter or just a broken fuel pump. Also a chance of a faulty coolant sensor. I've also heard that sometimes a faulty camshaft sensor won't show up on an OBD test, does anyone have any experience with this? Any thoughts? Please help!

 

Has the timing belt ever been changed?

 

No, the timing belt has never been replaced.

 

Well, it was supposed to be changed at 60,000 miles (and every 60,000 miles after that). If the belt breaks while driving, the cams stop spinning, the valves stop moving and the pistons usually hit the valves and cause all sorts of damage. I'd start by chacking the belt first.