Possible causes

– Transmission Fluid Pressure Sensor A harness is open or shorted
– Transmission Fluid Pressure Sensor A poor electrical connection
– Faulty Transmission Fluid Pressure Sensor A
– Transmission Valve Body

Tech notes

There is a service bulletin for the P0840 code for the following Nissan Models:

2007-2008 Nissan Altima
2007-2008 Nissan Rogue
2007-2008 Nissan Sentra

The bulletin suggest replacing the transmission valve body.

Nissan Factory Service Bulletin OBDII Code P0840

2007 Nissan Maxima

Nissan Factory Service Bulletin OBDII Code P0840

For Nissan vehicles with with CVT transmission, Nissan has extended the CVT warranty from 5 years/60,000 to 10 years/120,000 miles

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0840 NISSAN Description

The transmission fluid pressure sensor A (primary pressure sensor) detects primary pressure of Continuously Variable Transmission (CVT) and sends Transmission Control Module (TCM) the signal.

NISSAN Technical Service Bulletins for P0840 code