2003 Dodge Neon Check engine light on constantly, no engine

The car is a 2003 Dodge Neon, it's my best friend's car. The check engine light has been on (like 90% of the time) for over a year. She's taken it in 3 times and told it was the camshaft sensor causing it to come on. And each time they replaced it, it came back on within weeks. She ended up having the transmission replaced about 3 months ago, and they also told her the check engine light was due to the camshaft sensor, so they replaced it. She drove out of the parking lot, it came back on, so she took it back, and they told her the sensor was brand new and that's the only thing it could be. They then tried to reset the check engine light and that didn't work either. So she gave up. She's got to renew her tags, and her car won't pass emissions with the check engine light on. Last week, the car starts backfiring and making a knocking noise. Her cruise control does not work, either. So she takes it to a different place, and they fix those issues, tell her the camshaft sensor is causing the check engine light to come on, she explains that it's already been replaced 5 times, but they do it again anyway. She leaves, it's off, takes it to the emissions place fails, but they tell her she has to drive for a certain number of miles after sensors are reset before it'll pass emissions. So we drive around for about 25 miles, and the light's back on. Any idea what it could be BESIDES a camshaft sensor? Neither of us knows anything about cars, but since that stupid sensor has been replaced 5 times and the light is always on, it seems logical that just maybe something else could be solving the problem. And how do you get a mechanic to trust something besides the diagnostic tool which is telling him something is broken that doesn't actually seem broken?