Ridgeline will not shift down from overdrive at stop. Engine light and D light are on. Engine light is steady, D light flashes.

When the Torque Converter Clutch (TCC) isn't engaging or locks in engagement the light flashes a warning...The TCC solenoid may stick due to dirty transmission fluid...If the Check Engine Light
(CEL) is not lit have your transmission fluid changed...If it is lit the code is probably pointing at a defective TCC, Computer (PCM) or the clutch itself... you can take the vehicle to an auto parts store and have them read and define the trouble code for free...saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

There will be a code in memory which will indicate the problem. From you saying it flashes at cruise it is most likely your torque converter is not locking up as it should.

When was the last time you had the transmission filter and fluid changed? Sounds like the filter is clogged.

Clutches inside transmission need to be replaced
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In many cars, this light means the car wants to use the overdrive gear but it has been overridden by the driver. If you have an overdrive button, try switching it on and see if the light stops flashing.
Another reason for the light may be that the transmission cannot shift into overdrive due to a fault. To be sure, check your owners manual and let us know if you still have questions.

if it is flashing then it is a MIL. if it is on steady then it may mean that o/d is turned off. is it shifting fine?

Sounds like you've used the wrong type of transmission fluid for your trans.
Check in your owners manual.

probably the vehcile speed sensor

od switch is on the shift lever. Should go off everytime you restart vehicle. Does it flash or is it on steady?