Low coolant level CEL

[DDlighttruck]

'03 ISX CM870. Low coolant level CEL.

I flushed the coolant four times around May 2017. First three times with simple green as a cleaner, then the last time with Cummins coolant flush cleaner. There was oil in the coolant from the original engine that was replaced. The original coolant level sensor worked if you cleaned it and put it back in. I've been running green coolant since then.

The original sensor was replaced around the same time I did an EGR tune-up, maybe June or July. I had one coolant hose blow in the summer, and one heater hose blow in the fall. So I've kinda lived with the CEL, in my opinion it seems to take a while to burp, and adding coolant was just part of life.

I wasn't adding coolant for a month or two. Life was good, except the CEL. Used a multimeter on the sensor, according to Insite my sensor was outside the ohm range listed for a good sensor, cleaned it and put it back in. Ordered a new sensor, about a week went by until I could get the new sensor, CEL was back on. Changed the sensor 2-3 weeks ago, good up until last weekend when the CEL came back. Popped the hood it's showing coolant is still full.

Do I flush it completely again and start running red coolant, now that I think the leaks are fixed? I read that the red coolant is better, regardless.

Do I change the sensor again, or look at possibly wiring issues? Maybe pop the original one back in?

I just don't get this, it should be a simple fix......