Faulty transmission fluid pressure sensor/switch 'A' itself.; Damaged, corroded, or disconnected wiring or connector at the sensor.; Open or short circuit in the signal or ground circuit between the sensor and PCM.; Low transmission fluid level or degraded fluid affecting hydraulic pressure readings.; Internal transmission mechanical issues causing actual low pressure, though less common.; PCM internal fault or software issue, rare but possible.; Contamination or debris obstructing the sensor port.

DTC codes / P08xx · Transmission / P0840

P0840Transmission Fluid Pressure Sensor/Switch 'A' Circuit

Q: What does code P0840 mean?

Transmission Fluid Pressure Sensor/Switch 'A' Circuit. P0840 indicates that the powertrain control module (PCM) has detected an abnormal electrical condition in the circuit for Transmission Fluid Pressure Sensor/Switch 'A', which is used to monitor hydraulic pressure inside the transmission. The PCM compares the sensor's signal to expected values and sets this code when the voltage or resistance falls outside the normal range for a calibrated period.

P0840 indicates that the powertrain control module (PCM) has detected an abnormal electrical condition in the circuit for Transmission Fluid Pressure Sensor/Switch 'A', which is used to monitor hydraulic pressure inside the transmission. The PCM compares the sensor's signal to expected values and sets this code when the voltage or resistance falls outside the normal range for a calibrated period.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaTransmission

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) on the dashboard.
Often no symptoms besides the warning light, especially if the fault is intermittent.
Harsh or delayed transmission shifts as the PCM may default to a fail-safe strategy.
Transmission may enter limp mode, limiting vehicle speed and gear selection.
Erratic or inconsistent shift quality under varying loads.
In some cases, the transmission may slip or fail to engage certain gears.

Common causes, most likely first

Faulty transmission fluid pressure sensor/switch 'A' itself.
Damaged, corroded, or disconnected wiring or connector at the sensor.
Open or short circuit in the signal or ground circuit between the sensor and PCM.
Low transmission fluid level or degraded fluid affecting hydraulic pressure readings.
Internal transmission mechanical issues causing actual low pressure, though less common.
PCM internal fault or software issue, rare but possible.
Contamination or debris obstructing the sensor port.

How to diagnose and fix it

Verify transmission fluid level and condition; top off or replace fluid if necessary.
Perform a visual inspection of the sensor and its wiring for obvious damage, corrosion, or loose connections.
Use a scan tool to check live data for the pressure sensor signal and compare to specifications.
Test the sensor's electrical circuit with a multimeter to check for opens, shorts, or excessive resistance.
If the sensor is faulty, replace it and clear the code to see if it returns.
Check for technical service bulletins (TSBs) or software updates for the PCM that may address false settings.
If the code persists after sensor replacement, diagnose internal transmission pressure issues or PCM faults.

What the pressure sensor code hides

A P0840 is often set by a simple wiring problem rather than a failed sensor, so inspect the harness near heat sources and moving components before condemning the sensor.
Low fluid level or a clogged filter can cause actual low pressure that triggers this code, so always check fluid condition and level first.
The code can be intermittent and may not illuminate the MIL immediately; a history of pending codes can reveal the pattern.
Replacing the sensor without verifying the circuit can lead to a repeat failure if the underlying issue is a chafed wire or poor ground.
Some vehicles use this sensor for multiple functions, so a fault may also affect shift quality and torque converter clutch operation.

Can I keep driving with P0840?

If the vehicle drives normally, you can drive cautiously to a repair shop, but avoid heavy acceleration or towing. If the transmission is in limp mode or shifting harshly, limit driving to a short distance to prevent further damage.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.