Start by checking the "Possible Causes" listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector's pins.
There is a factory service bulletin for the following Nissan Models:
2007-2008 Nissan Altima
2007-2008 Nissan Rogue
2007-2008 Nissan Sentra
The bulletin suggests replacing the transmission valve body.
2007-2008 Nissan Altima, Rogue, Sentra Factory Service Bulletin OBDII Code P0840
2007 Nissan Maxima
2007 Nissan Maxima Factory Service Bulletin OBDII Code P0840
For Nissan vehicles with CVT transmission, Nissan has extended the CVT warranty from 5 years/60,000 to 10 years/120,000 miles
The cost to diagnose the P0840 NISSAN code is 1.0 hour of labor. The auto repair's diagnosis time and labor rates vary by location, vehicle's make and model, and even your engine type. Most auto repair shops charge between $75 and $150 per hour.
The Transmission Fluid Pressure Sensor 'A' (primary pressure sensor) detects primary pressure of Continuously Variable Transmission (CVT) and sends the Transmission Control Module (TCM) the signal. The Diagnostic Trouble Code (DTC) will set when the TCM detects no signal from the Transmission Fluid Pressure Sensor 'A'.
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