Do you have access to a scan tool that can read stored transmission codes? That will point you in the direction of what's causing the lack of overdrive.
Here are some causes for loss of overdrive in your transmission (considering you have a 4L60E tranny).
- Torque converter is failing to lockup, causing what could be mistaken as loss of overdrive. To test this out, drive on the highway and see if your RPM is jumping unsteadily with barely any changes to the gas pedal. If it does, your converter is failing to lockup.
- Transmission is in limp mode - overdrive doesn't work in limp mode and limp mode is what the transmission will operate in when a serious fault is detected.
- Problem with the linkage - the transmission linkage has a physical problem that's preventing it from going into overdrive. This would need to be corrected under warranty by the transmission shop.
- Overdrive band failure - the transmission band responsible for fourth gear fails overtime and can no longer engage the overdrive gear. If this band wasn't replaced with the rebuild, this can be the culprit.
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