B1503 code, under uphill load

2010 Prius 4 - purchased used Jan 2019 at 77,000 miles. No issues at all. Did full clean of intake, EGR, changed plugs to current spec, added catch can, changed transmission fluid. Oil and filter changed on average every 6000 with full synthetic Mobil 1 as I know I put more load on the engine than average. All fluids good and topped off. Awesome and not a single issue for the first 5-6 months since doing all that.

About every other week, I do a lot of heavy mountain climbing in the high Sierra Nevada mountains in
California. Usually 10-15 minutes or more of constant uphill under full load, from 1500 foot elevation to anywhere from 6000 to 8000 feet. 40-55mph. 15-25 miles of load. No problems at all, and still getting relative good mileage (average 45-50mpg for the 500+ mile round trips, downhill makes up for the burn up hill) No issues at all for the first 5 or so months. I do tack on a bunch of miles, as every other weekend I put on 500+ miles with these trips alone, and already have passed 92,000 mile since buying it. Rest of the time it's normal commute driving in Los Angels. , about 50-70 miles a day and zero issues.

Now I'm starting to get the high temp light on while under load after 5-10 minutes of heavy load . B1503 - Exhaust Heat Management Warning according to ODBI. No problem at all for months under the same load or heavier. And it's getting more and more sensitive, and triggering earlier. This weekend simply going up the Grapevine in CA triggered it at anything above 45 MPH. Double checked all fluids, they are fine, all fans are working no issues. No blockage to radiator. No other error codes, no triangle.. Once I back of to very slow speeds (almost dangerous since everybody else is still doing 50+) the light goes off. Had zero problems doing the same exact route for the first 5-6 months. Since the problem started had no major changes to vehicle other than constant maintenance - and other than in May I got the current recall update.. doubt that's related.. but putting it out there.

Any ideas as to why this is now triggering, and seemingly getting more and more sensitive? Dosn't effect my day to day driving, only the bi-monthly trips up into the mountains and heavy loads.

 

So sounds like I need to test the sensor. Forgot to mention I put in a brand new 12v battery when I bought it as it was still on the original and I didn't want to wait for any issues with it.