P0791Generic code

P0791 Code: Intermediate Shaft Speed Sensor A Circuit

The P0791 code means there is an electrical fault in the circuit of intermediate shaft speed sensor A in the transmission. The main effect is limp mode. Check the connector and wiring first, because they fail as often as the sensor.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$40 to $400
Difficulty
Medium
System
Transmission
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

On this page

What the P0791 code means

Some transmissions, mostly those with six or more speeds and dual-clutch units, have a sensor on an intermediate shaft so the computer can monitor more precisely which clutches are applied.

The sensor generates pulses whose frequency depends on the speed or the quantity being measured. A circuit fault, the P0791 code, can come from the sensor, the wiring or the power supply.

Without this signal the computer loses information it needs to control the shifts, and it can switch to limp mode. See the guide to the input/output speed sensors for how they are tested.

Always start with the simplest items: fuse, relay and connector. Then check power, ground and signal with the component connected (using back-probes), and finally the continuity of each wire up to the computer. Only if all of that is good should you replace the component.

P0791 code symptoms

  • Check engine light on.
  • Transmission limp mode.

Likely causes of the P0791 code

  1. Damaged or corroded connector. The most frequent cause.
  2. Wiring cut or shorted between the sensor and the computer.
  3. Failed sensor, open or shorted internally.
  4. Missing power reference. Check whether other sensors are also failing.
  5. Computer. Uncommon; rule out everything above first.

How to diagnose the P0791 code step by step

  1. Read the sensor value on the scan tool and compare it with what is expected.
  2. Inspect the connector for corrosion, water or loose pins.
  3. Check power and ground with the key on: a correct reference and less than 0.1 V of drop on the ground.
  4. Measure the signal with the connector plugged in and see whether it changes with the condition.
  5. Do the wiggle test on the harness with the scan tool in graphing mode.
  6. Check the continuity of each wire up to the computer (under 1 ohm).

How to fix the P0791 code

  • Repair the connector or the wiring.
  • Replace the sensor if the tests confirm it.
  • Correct the real condition, if there is one.
  • Clear the codes and verify with a road test.

Technical reference data

  • Most are 3-wire Hall effect sensors.
  • These are approximate values: check the manufacturer data for your vehicle.

Brand-specific notes

This code appears on Honda and Hyundai transmissions and on some dual-clutch units.

P0791 on Honda

It shows up on Honda transmissions that carry an intermediate shaft speed sensor.

P0791 on Hyundai

It also appears on Hyundai transmissions, where the same connector and wiring checks apply before the sensor is replaced.

P0791 on Dual-Clutch Transmissions

Some dual-clutch units use this sensor to monitor which clutches are applied, so a circuit fault here can send the transmission into limp mode.

Frequently asked questions about P0791

What does the P0791 code mean?
It is a generic OBD2 code that means there is an electrical fault in the circuit of intermediate shaft speed sensor A. The cause can be the sensor, its wiring or its power supply.
Can I drive with a P0791 code?
Yes, with caution. With a P0791 code the transmission may go into limp mode, so keep trips short until it is repaired.
Does the sensor have to be replaced?
Only if the tests confirm it. The connector and the wiring fail just as often.
What does the intermediate shaft sensor do?
Some transmissions, mostly those with six or more speeds and dual-clutch units, have a sensor on an intermediate shaft. It lets the computer monitor more precisely which clutches are applied.
How much does it cost to fix P0791?
As a rough guide, between $40 and $400.
Will a P0791 code clear itself?
It can go out if the connection was only loose or damp for a moment. A damaged connector, wire or sensor will bring it back.
What is the most common cause of P0791?
A damaged or corroded connector. Check it and the wiring before you buy a sensor.

Related codes

Questions about the P0791 code

There are no questions about this code yet. Tell us about your case: vehicle, symptoms and what you have already checked.