Hello everyone, just bought my first Infiniti 3 weeks ago. 2006 G35x Auto, 122k. It had a leaky radiator and was replaced yesterday, along with thermostat and both belts. I drove it afterwards for maybe 10-15 miles of city driving to shake out and burp the coolant, etc. Now today, the trans doesnt want to shift to 5th, then all other shifts seem labored and theres a very faint spooling/whining noise when about to or during shifts. We had to disconnect trans lines to radiator, but maybe lost a half quart and didnt replenish as the dip stick looks and now still looks fine while hot. Also, after doing a slowdown and reaccelerating in between highways, it shifted perfect all the way to 5th....until my next turn when it had to downshift, then it was labored and weird noise and no 5th again. Does anyone have any experience with this or could point me in the right direction? Thanks so much and am glad to now be a part of the community.

Get a clean container, disconnect the return trans fluid line from the transmission, aim it down into this clean container then fire the car up and verify it's actually pushing fluid back. You may have some blockage inside that new radiator. I've seen all sorts of thing happen including folks not removing the plastic thread covers from the radiator during installation.

I'd verify the transmission fluid level again, I strongly suspect you're actually low. Pull and wipe the dipstick at LEAST a half-dozen times until you verify an ACTUAL fluid level while the transmission is RUNNING and HOT, go for a drive to get it up to temp, don't turn the engine off, check fluid level while the vehicle is parked and the engine is RUNNING.

Thanks for the advice. I know for sure we took off the plastic caps on the radiator and put the hoses back on immediately to avoid contamination. I will try this method in the next few days as Id like to have more ATF on hand and only have one quart of Matic S right now. Another thing to note is there are no warning lights on dash through all of this. I am going to reset the TCU today and check, recheck fluid and add if necessary. The shifting issue seems worse the higher up in gears I go, too. 1-2 and 2-3 are normal, except for a sporadic hard downshift. I will check and try the method you suggested, though. Thank you very much.

Keep in mind you lost quite a bit of fluid simply from the act of replacing the radiator (all the fluid that was inside the radiator), in addition to anything that drained out of the lineset. You could be upwards of a quart low which is a LOT for an AT.

Check fluid levels so you know where you're at, you're not going to get a TCM code to throw until something breaks.