Transmission Fluid Pressure Sensor/Switch E Circuit Intermittent
The transmission fluid pressure sensor is sending an unstable or intermittent signal to the engine computer, like a loose electrical connection that keeps dropping out. This causes the transmission to lose confidence in pressure readings and may trigger limp mode or shifting issues.
The ECM monitors voltage output from the transmission fluid pressure sensor (Switch E) to verify hydraulic pressure is within safe operating range. The sensor should send a continuous, stable analog signal; intermittent drops or noise indicate a wiring fault, poor connection, or failing sensor. The ECM flags this when the signal becomes unreliable rather than completely absent.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage Signal | 0.5–4.5 V (steady, continuous) | Intermittent dropouts, noise, or signal loss |
| Pressure Range | 20–100 psi (depending on gear) | Erratic readings or sensor disconnections |
Code P0991 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0991 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.