P0994 Code: Transmission Fluid Pressure Sensor/Switch F Circuit Low

What the P0994 code means

Modern automatic transmissions measure fluid pressure in several circuits to confirm that each clutch actually receives pressure when its solenoid is commanded on.

The sensor gets a 5 volt reference and a ground from the computer and sends back a signal somewhere between about 0.5 and 4.5 volts. The P0994 code is a low signal, which is usually caused by the signal wire shorted to ground or by a missing reference voltage.

When the signal fails, the computer cannot confirm that the clutches are applying, and it usually puts the transmission in limp mode. On any transmission code, check the fluid first. The level has to be right using the manufacturer procedure, which often calls for the engine running at a specific temperature. The fluid should be red or pink and should not smell burnt. Low or worn fluid causes slipping, harsh shifts and solenoid or gear ratio codes that go away once the fluid is corrected.

To confirm the diagnosis, unplug the component with the key on and see what happens to the reading or the code. If the value jumps to the opposite extreme, the wiring is fine and the component is the part pulling the signal to ground. If nothing changes, go hunting for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0994 code symptoms

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

Likely causes of the P0994 code

1. Signal wire shorted to ground where the harness rubs against the engine, the chassis or a bracket.
2. Reference supply shorted. If several sensors fail at once, suspect the shared reference.
3. Damaged sensor with an internal short.
4. Connector with water or corrosion, which creates a path to ground.

How to diagnose the P0994 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 voltage drop on the ground should be under 0.1 volt.
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 the continuity of each wire to the computer. It should be under 1 ohm.
7. Check the fluid level and condition.

How to fix the P0994 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 the repair with a road test.

Technical reference data

• 5 volt sensor or pressure switch.
• These values are only a guide, so look up the manufacturer data for your vehicle.

Brand-specific notes

Check the service manual to see which pressure circuit matches the letter F on your transmission, because the lettering is not the same from one transmission to the next.

Frequently asked questions about P0994