Overdrive light flashes when I go up a steep grade

Normally the flashing OD light when pulling a grade is telling you that the torque converter is slipping and not able to lock like it is supposed too.

When you checked codes was your scan tool able to pull transmission codes? Unfortunately they are different so we need to make sure it is able to do that as I suspect you have a code like a P0733 which is for the torque converter clutch solenoid. Basically this is monitoring the input and output speed of the transmission so that it can tell if the converter is slipping.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Let's stop by a parts store and see if they can pull other codes that will point us in this direction.

If not, we are going to need to use a scan tool that will tell us what the input and output speed sensors are reading. Basically the input speed is engine speed but we need to see what the output speed is when the torque converter is locked up.

Let's start with this and go from there. Thanks

Sorry. I forgot to mention. Have you checked the fluid condition? If the fluid is burned then we need to change it but this also points to a clutch slipping as that is what causes the fluid to deteriorate. Heat from a slipping clutch burns the fluid and makes these issues worse.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Thanks

Okay. Then we need to double check codes with a scan tool that we know can read transmission codes. This flashing light is like a check engine light for the transmission only. It could be a sensor, circuit, or solenoid issue.

Again, I am betting a torque converter issue.

Can you get a video of what it is doing? You may be correct with this because seeing it/hearing it is different then just reading about it. Let us know what turns out with this if you can't get the video. Thanks