Intermediate Shaft Speed Sensor A Circuit
The P0791 code indicates that the Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected a malfunction in the circuit for the intermediate shaft speed sensor 'A'. The module sets this code when it fails to receive a consistent, valid rotational speed signal from this sensor, which is necessary for calculating proper shift strategies.
While the vehicle can usually still be driven, driving with this code for an extended period can lead to poor shift quality and potential internal transmission wear. It should be diagnosed and repaired soon to restore proper transmission function and prevent further damage.
Common symptoms
- Illuminated Check Engine Light
- Harsh, delayed, or erratic transmission shifting
- Transmission slipping between gears
- Speedometer functioning erratically or not at all
- Vehicle entering a fail-safe or 'limp' mode (locked in a single gear)
Likely causes
- Faulty intermediate shaft speed sensor
- Damaged, corroded, or loose wiring and connectors in the sensor circuit
- Dirty or contaminated transmission fluid affecting sensor operation
- Metallic debris built up on the sensor's magnetic tip
- Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM) (rare)
What to check before replacing parts
Check the transmission fluid condition and level
Look for: Low fluid levels, a burnt smell, or heavy metallic contamination in the fluid that could interfere with the magnetic sensor's ability to read the reluctor wheel.
Visually inspect the sensor wiring harness and connector
Look for: Frayed wires, melted insulation, corrosion in the connector pins, or a loose connection at the intermediate shaft speed sensor.
Remove and physically inspect the intermediate shaft speed sensor
Look for: Cracks in the sensor housing or a heavy buildup of metallic shavings on the magnetic tip that can distort the speed signal.
Test the sensor's internal resistance or output voltage using a digital multimeter
Look for: Resistance readings that fall outside the manufacturer's specified range, or a lack of AC voltage generation when the shaft is manually rotated.
Test the circuit continuity between the sensor connector and the TCM/PCM
Look for: High resistance, an open circuit, or a short to ground or power within the wiring harness connecting the sensor to the control module.
This page explains the generic P0791 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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