even though the glow bar will heat it doesn't necessarily meen it is good. a glow bar has to pull at least 3 amps to open solenoids to allow gas to pass through. if the glow bar pulls less than 3 amps it is bad even though it glows.
it can be checked with an amp meter, but i would say that is the problem. i have worked on appliances 32 years, 95% of the time this is the fix.

ok. it gets a little more complicated from here. need to check the flame sensor. the box mounted on the side of the heater. it has 2 terminals. AT ROOM TEMP! the sensor should read continuity, then set your ohm meter at a high scale such as 10-20k. place leads on terminals, cover glass on sensor with hand and read ohms. then put a heat source, such as lite bulb at sensor and read ohms. the readings should be a good distance apart. if readings do not vary or the sensor does not read continuity, the sensor is bad. if you could post a picture of the schematic wiring diagram it would be helpful.

yes you can remove the sensor. 1 or 2 screws depending on the model.