The fans will both turn on at the same time, both will be on low speed at the same time, both will be on high speed at the same time.

Verify both fans are pulling air into the engine bay through the radiator and that one of them isn't pushing air instead. You should also be able to visibly SEE the fans switch from low to high speed, let them turn on while idling the vehicle, then have someone else hit the air conditioner button which will force them into HIGH speed.

I suspect you have a failing or intermittently failing relay in the IPDM. There are 3 relays, if you aren't proficient in bench testing them then just replace all 3. They're not that expensive and it's one of those few components that can cost you an engine so it's better to just swap all 3 out for new ones rather than take a chance possibly overheating your car.

It's possible for a bad relay to cause only one side of the fans to turn on, if you have never seen the driver fan turn on, even on high speed forced from air conditioner, then you definitely have a problem.

Section EC - Engine Controls under the "engine overheat" section has all the stuff on the high/low fan operation.

https://www.nicoclub.com/infiniti-se...ls#g35_g37_q40

 

Replaced fan motors with TYC 631210 and all is good. Fit is good. They have long leads on them which I just zip tied to the shroud and maintain easy access to the connectors/terminals for future diagnosis (if needed) Quality appears to be good. Time will tell.

 

Thanks for info Cleric. Finally got to it today, turns out one of the fuses was cracked, relays still good in this case. the guess is that the fan that dismounted was stressing the fuse while it was pushing/grinding into the radiator.