Transmission Fluid Pressure Sensor/Switch "E" Circuit Intermittent
Diagnostic trouble code P0991 indicates that the Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected an intermittent or erratic voltage signal from the Transmission Fluid Pressure (TFP) Sensor/Switch 'E' circuit. This intermittent signal falls outside the expected parameters for the current operating conditions, suggesting a fault in the sensor, its wiring, or the hydraulic pressure it monitors.
While the vehicle may still be drivable, an intermittent transmission pressure sensor fault can lead to harsh shifting, slipping, and potential internal transmission damage if left unaddressed. It should be diagnosed and repaired promptly to avoid more costly repairs.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering 'limp mode' (restricted to a single gear)
- Decreased fuel efficiency
Likely causes
- Low or dirty transmission fluid
- Loose, corroded, or damaged wiring and connectors in the TFP sensor 'E' circuit
- Faulty Transmission Fluid Pressure Sensor/Switch 'E'
- Mechanical internal transmission failure affecting hydraulic pressure
- Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Fluid that is low, burnt, or heavily contaminated with debris, which can cause erratic pressure readings and sensor malfunctions.
Visually inspect the wiring harness and connectors for the TFP sensor 'E'.
Look for: Chafed wires, melted insulation, loose connections, or corrosion at the sensor connector that could cause an intermittent electrical connection.
Scan for other related transmission or pressure codes.
Look for: Additional codes that might indicate a broader electrical issue, a failing pump, or multiple sensor failures.
Test the TFP sensor 'E' circuit for continuity and resistance using a digital multimeter.
Look for: Intermittent open or short circuits when gently wiggling the wiring harness, or resistance values outside the manufacturer's specifications.
Monitor the TFP sensor 'E' data PID with an advanced scan tool while driving.
Look for: Erratic voltage or pressure readings that drop out or spike unexpectedly compared to actual transmission operation and commanded line pressure.
This page explains the generic P0991 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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