I have a 2018 F150 with 6000 miles and the tire pressure warning is going off at 34psi, which is 1psi below the recommended tire pressure of 35psi. My local dealer, Hilbish Ford, tells me it is designed by Ford to alarm at 1psi low and there is nothing they can do about.

I really can't believe this is true because the TPMS probably isn't accurate to +/1psi and normal daily tire pressure fluctuations are greater than that.

Is anyone aware of the TPMS alarm thresholds or able to recommend a solution?

 

They have come a long way with tire pressure monitoring systems and they are more specific regarding tire pressures than previous years.

Something you might consider, again depending on where you reside, .
If ambient temps don't fluctuate much you probably could get by with 35-36 psi since a lot of gauges are not always that accurate.
However, if you reside where temps go down quite a bit and the system is pretty critical it could set off the light until driven some and tires warm up. You most likely will not notice much of a difference if the press was 1-2 lbs over the trigger setting. Most recommended tire pressures are supposed to be checked when the tires are cold and not driven for 24 hours. So as you can see there is room for some flexibility.

 

I run my factory 265/60-18's at 40 PSI on my 2015 Lariat SuperCrew. My last trip to Arizona from New York this past September netted a running pressure between 42-43 PSI per the tire monitor system. Pre-trip ambient reading was 40 PSI for all 4 tires. They were all still reading ambien pressure of 40 PSI after reaching Arizona.