HELP?? my I35 not changing gears from 2-3

[purcellwr]

when I drive my I35 for an hour or so it loses power and wont accelerate. It runs fine up until that point. and when it happens the car just seems to be in neutral like coasting and if i give it gas it revs up but does not respond but when the speed decreases to about 20 mph i can fell the car responding to the gas but it still wont accelerate over 25-30 mph like it wont shift to 3rd gear. but if i stop and turn it off for a minute and restart it, it runs like nothings wrong. I had a check engine light fault code for a speed sensor and now that I've had it replaced the same code is still coming up and the problem remains. Does anyone have any suggestion
Modified by purcellwr at 7:26 PM 9/13/2008

[SteveTheTech]

Hey there and Welcome to NICO!

I would start by checking your transmission fluid level, and perform a paper towel test to look for clutch material and debris. * Make sure to check it with the engine warm and running in Park on flat groundWould it be possible to provide the DTC#? Are there any other symptoms?What's your recent service history?Does it happen after driving on the highway for 1+ hour, in TCC lockup?

[purcellwr]

I had actually three DTC codes P0720 output speed sensor circuit malfunction. I recently took it to INFINITI of raliegh to have it serviced and they supposedly clear two manufacture faults that where displaying about 8 months ago a p1574 and p1456 they put in a speedometer Assy the service engine light went out and within 6 months it came on again with the previous DTC. I checked the transmission fluid it looks good. I'm a novice at this so the paper towel test i'm not sure of what that entails. I have no other symptoms, when its running right it's accelerating fine, I feel as if I'm losing some gas mileage but its not so bad that I could say for sure. and most of my driving is local city driving. I'll drive around town stopping here and there and after an hour or so of actual on-the-road time it happens out of the blue. Ive tried to manually shift but it doesnt respond to that either. My routine maintenance has been pretty consistent. No idea what TCC lockup is.

[SteveTheTech]

It sounds to me like you have a connection issue somewhere.

Problem is that a PO1574 is not going to set a CEL. It usually sets with a CAN code. Meaning there is a connection issue somewhere, which would describe this incident. It sounds like you have a heat related intermittant pin fit issue. There are several connectors in this circuit and diagnosis for this is not easy for someone without experience. I strongly recommend taking it to the dealer

Below I added the information pertaining to what sets the two DTCs that you mentioned in case you were curious.

PO1456=EVAP controlComponent Description

The EVAP canister vent control valve is located on the EVAP canisterand is used to seal the canister vent.This solenoid valve responds to signals from the ECM. When theECM sends an ON signal, the coil in the solenoid valve is energized.A plunger will then move to seal the canister vent. The abilityto seal the vent is necessary for the on board diagnosis of otherevaporative emission control system components.This solenoid valve is used only for diagnosis, and usually remainsopened.When the vent is closed, under normal purge conditions, theevaporative emission control system is depressurized and allows“EVAP Control System (Small Leak)” diagnosis.

PO1574=Cruise control vehicle speed sensorComponent Description

ECM receives vehicle speed signals via two different lines, anddetects vehicle speed for ASCD control.Vehicle speed signals are input from combination meter and TCMseparately.Signal from TCM is sent via CAN communication line.