I noticed about 50,000 miles the radiator light would flicker on and off. The engine temperature alwys stayed in the normal region and the radiator was full of fluid. The Mazda repair guy told me that this is accuring in a lot of the RX-8s. There is a plastic waffer on the gauge ball inside the radiator that measures the fluid. When the waffer starts breaking up, the gauge ball starts fluctuating greater sending a signal that the radiator is low on fluid. But it actually is not. Cost $900.00+ to replace the entire radiator just for this tiny plastic waffer. Update - Amazon has replacement reservoirs that you can buy and DYI here is the link - Genuine Mazda N3H1-15-351G Radiator Reservoir Assembly https://www.amazon.com/dp/B0091U3CBO/ref=cm_sw_r_awd_8Ezdvb1PXEXR5 Only $110 OEM
I was told the same thing by my repair man at Mazda Mall of Ga. The light is always on now because I don't think I should have to pay for a defect. I was told the whole tank would have to be replaced ($600+) Not good!
MY RAD LAMP COMES ON AND OFF, CAR DOES NOT OVER HEAT.. VERY DISTURBING.. DEALER SAYS THEY DON'T KNOW WHAT IT IS, HAVE TO PAY CHECK OUT FEE.. I FEEL THAT THEY ALREADY KNOW WHATS UP.. BUT I AM A CAPTIVE AUDIENCE..
I guess I had the car for two or three years the first time this occurred - maybe about 50,000mi. Had it to the (stupid) dealership couple times and they couldn't tell me what the problem was or if it was serious. Finally told me it was probably a sensor and told me what it would cost to replace it. I said wow! but replace it. They called and told me parts orders were backed up and it would be weeks before they could get part in. I told them forget it! Figured if it was a faulty sensor, the information was false anyway. Finally, when I was stranded at the mall with an overheated car, I took it to a reliable foreign car repair guy in town and he got the reservoir and repaired the car for me right away!
Now at 119,000 the light is starting to go off and on again. I sure don't want to get stranded again, so guess I will call my guy and see about getting the SAME PROBLEM FIXED AGAIN. Seems like a poor design!