P0991
Transmission Fluid Pressure Sensor/Switch E Circuit Intermittent
Powertrain Transmission Control Pressure Sensor Intermittent 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Transmission shifting harshly or erratically
Check Engine Light illuminates intermittently
Transmission stuck in limp mode or safe mode
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Wiring harness connector at sensor
Inspect and reseat the connector at transmission fluid pressure sensor E; clean corrosion with contact cleaner and push firmly until it clicks.
2
Transmission fluid pressure sensor (Switch E)
Replace the sensor if corrosion or internal wear is visible; follow OEM torque specs when installing.
3
Transmission wiring harness
Check for frayed or pinched wires between sensor and transmission control module; repair or replace damaged sections.