P0840 Code: Transmission Fluid Pressure Sensor A – Fix – iCarsoft Official Store

P0840: Trans Fluid Pressure Sensor A Range/Perf

ACTIVE DTC GUIDE · P0840 · Transmission / Fluid Pressure Sensor

⚡ Quick Reference — P0840

DTC CodeP0840

DefinitionTransmission Fluid Pressure Sensor / Switch A — Range/Performance

SeverityHIGH — Shift Quality & Limp Mode Risk

SystemAutomatic Transmission / TCM

Typical Repair Cost$100 – $800

Diagnostic ToolOBD-II + TCM live pressure data

1What Does P0840 Mean?

P0840 sets when the TCM determines that the signal from Transmission Fluid Pressure Sensor/Switch A is outside the expected range or does not respond correctly to commanded pressure changes. This sensor monitors hydraulic line pressure inside the transmission valve body — the pressure that actuates clutch packs and bands to produce gear changes. Normal line pressure ranges from 55–90 psi at idle up to 180–250 psi at full throttle. A sensor reading that stays fixed, reads below 20 psi at speed, or fails to rise with throttle input triggers P0840. The TCM may enter limp mode to prevent clutch damage from undetected low-pressure conditions.

Transmission fluid pressure sensor A location on automatic transmission case near valve body with electrical connector

Fig 1. Transmission Fluid Pressure Sensor A — typically threaded into the transmission case near the valve body. Check wiring before condemning the sensor.

2Most Affected Vehicles

Nissan CVT (RE0F10A / RE0F11A)2013–2018 Nissan Altima 2.5L CVT
2014–2019 Rogue 2.5L CVT
Pressure sensor failure from CVT fluid contaminationVERY HIGH INCIDENCE

GM 6T70 / 6T75 (Buick, Cadillac)2010–2016 Buick LaCrosse 3.6L
2010–2015 Cadillac SRX 3.6L
Pressure sensor circuit corrosionHIGH INCIDENCE

Chrysler / Dodge (62TE / 45RFE)2007–2016 Chrysler Town & Country
2007–2012 Dodge Ram 1500 (45RFE)
Switch-type sensor failure from fluid degradationMODERATE–HIGH

3Root Causes

Failed Pressure SensorPiezoresistive sensor element fails internally. Signal stuck at high or low voltage regardless of actual pressure. Most common cause on vehicles over 100,000 miles.
Low Transmission Fluid LevelLow fluid causes actual low line pressure — the sensor reads correctly but the pressure is genuinely insufficient. Check fluid level first before diagnosing the sensor.
Transmission Fluid DegradationBurned or contaminated fluid affects hydraulic viscosity, causing pressure fluctuations that the sensor reads as out-of-range. Fluid service often resolves P0840 on Nissan CVT platforms.
Wiring or Connector FaultCorroded sensor connector or damaged harness causes signal drop-out. Perform voltage drop test on the 5V reference and signal wires at the sensor connector.
Internal Transmission FaultWorn pump, stuck pressure regulator valve, or leaking clutch circuits can cause genuine low pressure. If sensor and wiring test good, hydraulic pressure testing is needed.

4Technical Specifications

📊 Transmission Fluid Pressure Sensor A Reference

Sensor reference voltage (5V supply)4.9 – 5.1 V

Signal voltage — low pressure (idle, in gear)0.5 – 1.2 V (~55 psi)

Signal voltage — high pressure (WOT)3.5 – 4.5 V (~220 psi)

P0840 trigger — out of range (high)> 4.8 V (open circuit)

P0840 trigger — out of range (low)< 0.2 V (shorted to ground)

Normal line pressure — idle in Drive55 – 90 psi

Normal line pressure — WOT180 – 250 psi

🔬 MECHANIC'S INSIGHT

On Nissan CVT-equipped vehicles (Altima, Rogue, Sentra), P0840 is almost always preceded by shudder on acceleration and sluggish response under load. Before replacing the pressure sensor, drain and refill CVT fluid with genuine Nissan NS-3 or NS-2 fluid — not generic CVT fluid. In approximately 40% of Nissan CVT P0840 cases, degraded fluid causing incorrect hydraulic viscosity is the root cause. Non-genuine fluid will not resolve the fault and can accelerate CVT wear. Check NHTSA for applicable CVT technical service bulletins before quoting sensor replacement.

5Diagnostic Steps

1

Check Fluid Level & ConditionVerify ATF/CVT fluid level and color. Dark fluid or burnt smell indicates overheating. Perform a fluid change with OEM-specified fluid as first step on Nissan CVT platforms.
2

Live Pressure Data MonitorUse iCarsoft CR Ultra P to monitor transmission line pressure in live data. A pressure reading that doesn't rise with throttle input or stays fixed at 0/maximum confirms sensor failure or genuine low-pressure condition.
3

Sensor Circuit TestWith key on, engine off: measure 5V reference and signal voltage at the sensor connector. No 5V reference = wiring fault to TCM. Fixed signal voltage regardless of conditions = sensor failure.
4

Hydraulic Pressure TestIf sensor and wiring test good, install a mechanical pressure gauge at the line pressure test port. Compare mechanical reading to sensor reading in live data. Mechanical pressure within spec = sensor failure. Low mechanical pressure = internal transmission fault.
5

Replace Sensor & VerifyInstall OEM pressure sensor. Clear codes with iCarsoft CR Ultra P. Monitor live pressure data during road test — verify pressure rises proportionally with throttle and transmission shifts normally through all gears.

Recommended Diagnostic Tool iCarsoft CR Ultra P Real-time transmission line pressure monitoring with TCM live data graphing — the definitive tool for differentiating P0840 sensor failure from genuine hydraulic faults on Nissan CVT, GM 6T70, and Chrysler transmissions.