OBD-II Code P0840 — What It Means & How to Fix It

What Does This Error Mean?

P0840 means the Transmission Fluid Pressure Sensor Circuit (Bank 1) has a fault. This sensor monitors the hydraulic pressure inside the automatic transmission. The TCM uses this reading to confirm gear changes happen at the correct pressure. When the circuit fails, the TCM cannot verify proper pressure and sets this code. You may experience erratic shifting, transmission slipping, or limp mode activation.

Affected Models

All 1996+ vehicles with automatic transmissions that have a pressure sensor
Common in GM vehicles with 6L80 and 6L90 transmissions
Common in Ford vehicles with 6F35 and 6R80 transmissions
Common in Chrysler vehicles with 68RFE and NAG1 transmissions

Common Causes

Failed transmission fluid pressure sensor with an open or short internal circuit
Low transmission fluid causing genuinely low pressure that triggers the sensor
Corroded or damaged sensor connector and wiring harness
Blocked oil passage preventing fluid from reaching the sensor port
Internal transmission wear causing actual low line pressure in the hydraulic circuit

How to Fix It

Check the transmission fluid level and condition immediately. Low fluid directly causes low pressure and triggers P0840.

If the fluid is low, look for a leak before simply topping it up. Leaks can rapidly deplete fluid and damage the transmission.
Inspect the pressure sensor connector for corrosion, moisture intrusion, or damaged wiring. Clean and reseat the connector.

The sensor is typically threaded into the transmission case or valve body and has a 2- or 3-wire connector.
Test the sensor with a multimeter. Compare the sensor's output voltage to the specification in the service manual across the pressure range.

A failed sensor often reads 0V or 5V constantly instead of varying with actual pressure.
Replace the pressure sensor if it is out of specification. This is typically a $50 to $150 part and simple to replace.

Always use a new sealing washer or O-ring when reinstalling a pressure sensor to prevent fluid leaks.
If the sensor tests good, perform a mechanical line pressure test using a gauge and an adapter. Low actual pressure confirms internal wear.

A professional pressure test can identify whether the pump, valve body, or clutch pack is causing the pressure drop.

When to Call a Professional

Pressure sensor replacement typically costs $150 to $400 parts and labor. If low actual line pressure is confirmed, a valve body or pump repair costs $600 to $1,500. A full transmission rebuild for severe pressure loss is $2,500 to $5,000. Always confirm with a mechanical pressure test before approving major work.

Frequently Asked Questions

Is P0840 dangerous to drive with?

It can be. If the transmission has genuinely low pressure, continued driving causes clutch pack damage. Check the fluid level right away. If the fluid is fine, it is likely just the sensor — still fix it soon.

How do I know if it is the sensor or real low pressure?

A mechanical pressure gauge test confirms actual line pressure. If the gauge shows normal pressure but the code persists, the sensor or its wiring is faulty. If pressure is genuinely low, the transmission has a deeper problem.

Can low transmission fluid trigger P0840?

Yes — low fluid is one of the most common causes. Check the dipstick or fill port first. Top up with the correct ATF and retest before replacing the sensor.