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GTECH Calibration
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P1791

TCM A/T Vehicle Speed Sensor Circuit Fault.

  • Honda
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
7
AreaTransmission
91
FaultThe specific fault in that area

What it means

On a Honda, P1791 means the transmission control logic has found a fault in the automatic transmission vehicle speed sensor circuit. The control unit saw a missing, implausible or erratic road speed signal when compared with other speed inputs, such as engine speed or the other transmission speed signals. The exact sensor layout and which unit sets the code depends on the model and gearbox.

Symptoms

  • Gearbox warning light or D indicator flashing on the dashboard
  • Harsh, delayed or erratic gear changes
  • Gearbox stuck in one gear or in a limp-home mode
  • Speedometer erratic or not working
  • Engine warning light on
  • Torque converter lock-up not working, with higher revs at cruising speed

Possible causes

  • Faulty or contaminated vehicle speed sensor or transmission output speed sensorlikely
  • Damaged wiring, chafed loom or poor connection at the speed sensor pluglikely
  • Corroded or loose connector pins at the transmission control unit or powertrain control modulepossible
  • Low or contaminated transmission fluid with metal debris on the sensor tippossible
  • Damaged reluctor ring or drive gear that produces the speed signal, depending on the modelrare
  • Faulty transmission control unit or powertrain control modulerare

How to diagnose

  1. Read all stored codes and freeze frame data, noting road speed, gear and engine speed when the fault set, and check for related speed sensor codes
  2. Check the transmission fluid level and condition using the correct procedure for the model, looking for burnt smell or debris
  3. Inspect the speed sensor, wiring and connector for damage, oil leaks, corrosion, heat damage or chafing
  4. Compare the live speedometer reading with the true road speed, and check the live data from each speed sensor while driving or with the drive wheels raised safely
  5. Check the sensor supply, signal and earth circuits, and test sensor resistance or output against the manufacturer's specification for that model
  6. Wiggle test the harness with live data running to find any intermittent open or short circuit
  7. Remove the sensor, if accessible, and check for metal contamination and for damage to the sensor tip and its reluctor
  8. Check for software updates or technical bulletins for the model and fit any calibration update only if it is a known fix

How to fix

  • Replace the faulty speed sensor and seal, with the correct fluid top-up if any was lost
  • Repair or replace damaged wiring and clean or replace corroded connectors
  • Correct the fluid level, or service the transmission with the specified fluid if it is contaminated
  • Replace a damaged reluctor ring or drive gear after confirming the cause of the damage
  • Replace or reprogram the control unit only after the sensor and circuit have been proven good, and only if the manufacturer lists this as a repair

Can I keep driving?

Medium severity

The car can usually be driven carefully a short distance, but the gearbox may be in limp-home mode and shifts may be harsh. Get it checked soon to avoid more wear or damage to the transmission.

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

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