Erratic Idle
 

2013 FRS, 110k, runs normal til you let off the gas at say, a stop sign, then idle goes to 1500 rpm for maybe 3 or 4 seconds, then dips to 500 rpm for a few seconds, before rising back to where it normally lives, 800- 950 rpm. No driveability issues, just the strange idle. No CEL's.

I am gonna guess vacuum leak or dirty throttle body.

AT or MT? (transmission)

Inspect/clean/replace intake air filter, inspect/clean MAF sensor, check records for last spark plugs replacement

Try without AC on.

this happens with a lot of early models, mine included. the stock tune on these early cars is not great. I fixed mine with Stage1 tune when my vehicle was stock and that got rid of the erratic idle. (about two weeks later when my headers arrived I went to a Stage 2+ tune).

if you are not planning on any tuning mods to your car you should be able to get your Toyota dealer to reflash your ECU to fix it https://jalopnik.com/heres-how-to-fi...ngines-5948647