Just changed engines with a 28,000 mile from a roll over. 2005 RX8 and radiator lite is on as soon as I get up to speed on hwy. Coolant is fresh and level correct. Fresh oil and correct level. Cat seems OK. new water pump and thermostat. Dash gauge in middle and doesn't move. Any ideas? Could it be the float in plastic tank is stuck? Engine runs great to 9500rpms and is strong.
btw what year did Mazda put a windage tray in oil pan?

 

I would say yes it's the coolant LVL sensor. Unplug it if it bugs you. But should be replaced ASAP.

 

Red line,
I had the same when I bought my 2004 RX-8. Here’s what I learned...
Early RX-8’s have a sensor problem where the float density changes over time and as the coolant temp rises the coolant expands meaning the density drops. So essentially the float becomes heavier than the coolant when it is warm allowing it to sink.
I read somewhere that Mazda reformulated the float material, but the float is inside the tank and cannot be replaced without destroying the expansion tank.
So you can buy a new tank from the dealer or there are other aftermarket upgrade options.
rx8performance sells a welded metal tank as a OEM compatible replacement.
Racing Beat sells an instrument cluster add on with gauges for oil pressure and oil and water temps.
Good luck!

 

Just get a new coolant reservoir tank. It's not difficult to replace, but you have to watch so you don't break the "nipple" going into the radiator, which can be brittle on an older car.

 

Thank you. I'll order a new tank.

 

Mine has been unplugged for years. I check oil and coolant lvls daily. Also all my coolant hoses and rad are new ( a year or two ago).

 

Thanks
al