P0991 Code: Transmission Fluid Pressure Sensor/Switch E Circuit Intermittent

What the P0991 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 P0991 code is an intermittent fault, and intermittent dropouts almost always come from a poor connection or a damaged wire.

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.

Intermittent faults are found by reproducing them. With the engine running and the scan tool in graph mode, move the wiring harness section by section, tap the connector gently and, if needed, warm the area with a hair dryer or wet the connector with a spray bottle. The spot where the reading jumps is where the fault is.

P0991 code symptoms

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

Likely causes of the P0991 code

1. Poor contact in the connector, with loose or backed-out pins.
2. Wire broken inside its insulation, which opens the circuit with vibration.
3. Harness rubbing against moving or hot parts.
4. Sensor with an intermittent fault, often tied to temperature.

How to diagnose the P0991 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 P0991 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 E on your transmission, because the lettering is not the same from one transmission to the next.

Frequently asked questions about P0991