Hello!
I have a 2017 toyota yaris iA with 38,000 miles and an automatic transmission. Occasionally, when I am on a stop light on drive or reverse after the engine has warmed up the RPMs drop to 500 and the car starts shaking. It does not appear like the car will stall or anything and the check engine light does NOT turn on. After I press on the gas pedal it runs normally. I took it to a mechanic which performed a throttle body cleaning, mass sensor cleaning, changed the spark plugs, and oil filter but this problem is still going on. No codes appeared with a basic scan tool either. I will also change the air filter but the current one is still not very dirty and I vacuumed it. What could it be?
Willing to bet its the egr system if it only happens after its warmed up.
So maybe it would be a good idea to clean the EGR valve?
And try to see if its working properly. It should hold vacuum and the diaphragm will open and close if you suck on the vacuum hose.
If its electrical you can measure the resistance when you change the vacuum pressure with a voltmeter.
Both while its off the car.