P0791
Intermediate Shaft Speed Sensor A CircuitP0791 is a generic OBD-II powertrain diagnostic trouble code: Intermediate Shaft Speed Sensor A Circuit. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0791 means
P0791 is stored when the TCM or PCM detects that the signal from Intermediate Shaft Speed Sensor "A" is absent, implausibly low, or erratic. The intermediate shaft — also called the counter shaft or secondary shaft — sits between the input shaft and the output shaft in dual-clutch (DSG/DCT), CVT, and certain planetary automatic transmissions. Its speed is used by the TCM to calculate actual gear ratio, monitor clutch slip, and time shift events precisely; without a valid signal, the module cannot execute adaptive shift control and must fall back to conservative defaults or limp mode.
On dual-clutch and DCT units, this sensor is especially critical because the two sub-transmissions share speed measurements to pre-select the next gear. Loss of the intermediate shaft signal causes prolonged engagement delays, harsh ratio changes, and — on some platforms — complete refusal to engage certain ratios. CVT applications rely on the signal to validate the belt/chain ratio and prevent overspeed; a missing signal may cause the TCM to clamp the variator to a fixed ratio. Planetary automatics use it to confirm the planetary carrier speed for shift-quality adaptation.
Diagnostic work typically focuses on the sensor's tone wheel, wiring harness (which routes through a transmission-fluid-filled environment), and TCM input circuit. Resistance checks (normally 300–600 Ω for passive magnetic sensors), oscilloscope waveforms, and harness continuity tests usually isolate the fault without requiring transmission disassembly.
Common causes
Most-frequently reported root causes when P0791 is logged.
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1
Faulty intermediate shaft speed sensor with a worn or shorted internal coil.
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2
Damaged tone wheel (reluctor ring) with missing teeth or ferrous debris bridging the gap.
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3
Chafed, corroded, or open sensor wiring harness inside the transmission or at the external connector.
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4
Corroded or pushed-back connector pins at the sensor or TCM reducing signal amplitude.
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5
Transmission fluid contamination or moisture ingress into the sensor body.
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6
Failed TCM input circuit (blown fuse, damaged input stage) unable to read the sensor signal.
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7
Software mismatch between sensor pulse-count specification and TCM calibration after a unit swap.
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8
Excessive bearing wear causing sensor air-gap variation and intermittent signal dropout.
Symptoms drivers notice
How to diagnose P0791
A typical diagnostic flow when this code is present.
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1
Retrieve all stored DTCs and freeze-frame data; note vehicle speed, shaft speed values, and any companion speed-sensor or ratio codes.
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2
Monitor live intermediate shaft speed data during a test drive; the value should rise smoothly from 0 Hz at rest to several hundred Hz at road speed.
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3
Inspect the sensor wiring harness and connector for chafing, corrosion, or damage where the harness passes through or alongside the transmission.
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4
Measure sensor resistance at the connector (typically 300–600 Ω for passive magnetic types; active sensors require a reference voltage check).
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5
Check harness continuity and ground integrity from the sensor connector back to the TCM.
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6
Inspect the sensor mounting and tone-wheel gap for debris, runout, or damaged reluctor teeth.
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7
Verify TCM software is current; flash available calibration updates before condemning the TCM input circuit.
Related powertrain codes
- P0700 — Transmission Control System Malfunction
- P0701 — Transmission Control System Range/Performance
- P0702 — Transmission Control System Electrical
- P0703 — Torque Converter/Brake Switch B Circuit Malfunction
- P0704 — Clutch Switch Input Circuit Malfunction
- P0705 — Transmission Range Sensor A Circuit malfunction (PRNDL Input)
Frequently asked questions
Which transmissions are most commonly affected by P0791?
Dual-clutch (DSG/DCT) units, CVTs, and multi-ratio planetary automatics with counter-shaft architectures are most commonly affected because they rely on intermediate shaft speed to manage pre-selection and ratio verification.
Can I drive with P0791?
If the transmission is in limp mode, only limited driving is advisable. Harsh shifts caused by missing speed data can accelerate clutch and synchroniser wear, so prompt diagnosis is recommended.
Will replacing the sensor always fix P0791?
Not always. If the tone wheel is damaged, the harness is broken, or the TCM input circuit has failed, a new sensor will not resolve the fault. Follow a full electrical and mechanical inspection before replacing parts.
How is P0791 different from input shaft or output shaft speed sensor codes?
P0791 is specific to the intermediate (counter) shaft that runs between the input and output shafts. Input shaft codes (e.g., P0715) affect the turbine/primary shaft speed; output shaft codes (e.g., P0720) affect the shaft driving the driveline. Each provides a different speed reference for the TCM's ratio and slip calculations.
Disabling P0791 in software
RaceTune can permanently disable P0791 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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