P0840 Code: Transmission Fluid Pressure Sensor/Switch A Circuit

What the P0840 code means

Modern automatic transmissions measure fluid pressure in one or more circuits (line pressure, clutch circuits) with sensors or switches. That is how the computer confirms that each clutch is getting pressure when its solenoid is commanded.

The sensor gets a 5 V reference and a ground from the computer and sends back a signal of roughly 0.5 to 4.5 V. The P0840 code is set when something is wrong with that circuit, and the cause can be the sensor, the wiring or the power supply.

When the signal fails, the computer cannot confirm that the clutches are applying correctly and usually triggers limp mode. Low or degraded fluid can cause truly low pressure, so rule that out first. On any transmission code, the first check is the fluid: the correct level using the manufacturer procedure (often with the engine running and the fluid at a specific temperature), a red or pink color and no burnt smell. Low or degraded fluid causes slipping, harsh shifts and solenoid or gear ratio codes that disappear once it is corrected.

Always start with the simplest things: fuse, relay and connector. Then check power, ground and signal with the component plugged in (using back-probes), and finally the continuity of each wire to the computer. Only replace the sensor when everything else checks out.

P0840 code symptoms

• Check engine light on.
• Transmission in limp mode.
• Harsh shifts or no shifts at all.
• Transmission stuck in one fixed gear.

Likely causes of the P0840 code

1. Damaged or corroded connector. This is the most frequent cause.
2. Wiring cut or shorted between the sensor and the computer.
3. Bad sensor, open or shorted inside.
4. Missing power reference. Check whether other sensors are also failing.
5. Computer. This is uncommon, so rule out everything above first.

How to diagnose the P0840 code step by step

1. Read the sensor value on the scan tool and compare it with what you expect.
2. Inspect the connector for corrosion, water or loose pins.
3. Check power and ground with the key on: the reference should be correct and the ground voltage drop should be under 0.1 V.
4. Measure the signal with the connector plugged in and see whether it changes with the condition.
5. Do a wiggle test on the harness with the scan tool in graph mode.
6. Check continuity of each wire to the computer (under 1 ohm).
7. Check the fluid level and condition.
8. Run a pressure test with a gauge if the transmission has a pressure test port.

How to fix the P0840 code

• Repair the connector or the wiring.
• Replace the sensor if the tests confirm it is bad.
• Correct the real condition if there is one.
• Clear the codes and verify with a road test.

Technical reference data

• 5 V sensor (0.5 to 4.5 V) or pressure switch (open/closed).
• Reference values are only a guide, so look up the manufacturer data for your vehicle.
• Wire continuity should be under 1 ohm, and ground voltage drop under 0.1 V.

Brand-specific notes

On many transmissions the pressure switches sit in the valve body, so fluid condition and the wiring inside the pan matter as much as the sensor itself.

P0840 on GM

On many transmissions the pressure switches are in the valve body, for example in the GM 6T.

P0840 on Aisin

On many transmissions the pressure switches are in the valve body, for example in Aisin units.

Frequently asked questions about P0840