Faulty output speed sensor 2 giving a spurious or noisy signal; Damaged, chafed or corroded wiring between the sensor and the transmission control unit; Poor connection at the sensor or gearbox plug (water ingress, corrosion, bent or loose pins); Metal debris or contamination on the sensor tip or the pulse wheel; Damaged or cracked sensor pulse wheel or tone ring; Low, degraded or incorrect transmission fluid affecting the gearbox and sensor readings; Faulty transmission control unit or mechatronic unit.

P1799

Output Speed Sensor 2 Circuit RPM Too High.

Volkswagen
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
7: AreaTransmission
99: FaultThe specific fault in that area

What it means

On Volkswagen, P1799 means the transmission control unit has seen an implausibly high signal from output speed sensor 2 circuit. The reported output shaft speed is higher than the control unit expects compared with other speed inputs, such as the input speed, engine speed or wheel speed signals. The fault may come from the sensor, its wiring or interference rather than a real mechanical overspeed. Which sensor is 'number 2' and where it sits depends on the gearbox type.

Symptoms

Gearbox warning light or malfunction message on the dash
Transmission limp mode, with the gearbox stuck in one gear
Harsh, delayed or erratic gear changes
Speedometer or rev reading erratic or dropping out, on some models
Engine management light on with a stored transmission fault
Reduced performance or poor economy due to the gearbox not shifting normally

Possible causes

Faulty output speed sensor 2 giving a spurious or noisy signallikely
Damaged, chafed or corroded wiring between the sensor and the transmission control unitlikely
Poor connection at the sensor or gearbox plug (water ingress, corrosion, bent or loose pins)likely
Metal debris or contamination on the sensor tip or the pulse wheelpossible
Damaged or cracked sensor pulse wheel or tone ringpossible
Low, degraded or incorrect transmission fluid affecting the gearbox and sensor readingspossible
Faulty transmission control unit or mechatronic unitrare

How to diagnose

Read all stored fault codes and freeze frame data, and note whether other speed sensor or plausibility faults are stored alongside P1799.
Check live data for output speed sensor 2 against vehicle speed, input speed and wheel speed, and see whether the reading jumps or spikes.
Inspect the sensor wiring and gearbox connector for chafing, oil contamination, corrosion, water and pin damage.
Check the transmission fluid level and condition using the manufacturer's procedure for your gearbox type.
Wiggle test the harness with the engine running and watch the live data for dropouts or spikes.
Check the sensor for debris on the tip and the mounting condition, and inspect the pulse wheel if accessible.
Test the sensor circuit for resistance, shorts and voltage against the wiring diagram for your model and gearbox.
Once wiring and sensor are ruled out, have the control unit and mechatronic unit checked by a specialist.

How to fix

Replace the output speed sensor 2 if it is faulty or contaminated.
Repair or replace damaged wiring and any corroded or loose connector terminals.
Clean debris from the sensor and its mounting area, and replace a damaged pulse wheel.
Correct the transmission fluid level or service the fluid to the manufacturer's specification.
Repair or replace the control unit or mechatronic unit only after the sensor and circuit are confirmed good.

Can I keep driving?

Medium severity

Short, gentle journeys may be possible if the gearbox is shifting normally, but get it checked soon. If the car is in limp mode, shifting harshly or slipping, limit driving to avoid gearbox damage.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.