Why won't my highlander shift in to 4th gear?

PO138-- Faulty Rear Heated Oxygen Sensor Bank 1 - Rear Heated Oxygen Sensor Bank 1 harness is open or shorted - Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection - Intake air leaks - Exhaust gas leaks - Inappropriate fuel pressure - Faulty fuel injectors Replacing the Rear Heated Oxygen Sensor Bank 1 usually takes care of the problem. PO606- Faulty Engine Control Module (ECM) - Engine Control Module (ECM) Harness power supply circuit is open or shorted - Engine Control Module (ECM) Programming Check the ECM fuse, if fuse is OK try erasing the code first and if the code comes back you may have to reprogram or replaced the ECM.

Thanks Reelin68. Do you think replacing O2 sensor combined with either clearing P0606 code or reprogramming ECU can eliminate shifting issue? I am really hoping I don't have to replace the unit. I heard it can cost a couple thousand dollars!! Can a used unit suffice if that is required?

Not sure if that will cure your shift issue. It may. If code persists, you will have to try reprogramming it or getting another. I would try Ebay or local salvage yard. Or you can remove your ECU and send it to these guys and they send you back a fresh rebuild for $189. SIA Electronics Toyota ECM ECU PCM Toyota Engine Computer Repair & Return SIA Electronics has many other repair services available. (800)737-0915 This listing is a REPAIR for your Toyota ECM/ECU/PCM. All Models and Years.

No, replacing O2 sensor will not help the shift issue. It will repair a fuel trim issue.In return give you a negligible gas milage and power gain should you decide to repair the transmission. There is a low probability that the shift solenoid will repair it. More than likely the $3500 boat anchor attached to your engine needs to be rebuilt, or replaced.