Was the recall procedure performed on your car? Toyota called back 2013-2015 Avalons to fix the sensor due to false alarms. If that work wasn't done, having it fixed now might take care of your issue. Here's a link explaining the problem.

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I had a 2013 Avalon and after the recall work was done I had constant false alarms of the PCS system. The dealer told me there was nothing wrong with the system and it was calibrated properly, but the false alarms continued. This may have been the first year of the PCS system on Avalons and I never felt it was calibrated properly. I recently purchased a 2017 Avalon and I've had no issues with the PCS system.

Btw- When I got the recall notice on the 2013 it said to disable the PCS system (until the recall fix) by pushing the PCS System button in the glove box. When depressed the PCS indicator on the instrument panel lights up. I assume that not the 'warning light' you are seeing.