P0856 Code: Traction Control Input Signal

What the P0856 code means

The traction control system (or the ABS) sends the transmission computer a signal when it is working, so the transmission can avoid shifting while the wheels are slipping.

It is a simple switch-type signal, so the computer sees either a high or a low voltage. The P0856 code is set when something is wrong with that circuit, and the cause can be the sensor, the wiring or the power supply.

When the signal fails, the transmission may shift at bad moments during wheel slip. Also check for ABS and traction control codes.

Always start with the simplest things: fuse, relay and connector. Then check power, ground and signal with the component plugged in (using back-probes), and finally the continuity of each wire to the computer. Only replace the component when everything else checks out.

P0856 code symptoms

• Check engine light on.
• Shifts during wheel slip.

Likely causes of the P0856 code

1. Damaged or corroded connector. This is the most frequent cause.
2. Wiring cut or shorted between the sensor and the computer.
3. Bad sensor, open or shorted inside.
4. Missing power reference. Check whether other sensors are also failing.
5. Computer. This is uncommon, so rule out everything above first.

How to diagnose the P0856 code step by step

1. Read the sensor value on the scan tool and compare it with what you expect.
2. Inspect the connector for corrosion, water or loose pins.
3. Check power and ground with the key on: the reference should be correct and the ground voltage drop should be under 0.1 V.
4. Measure the signal with the connector plugged in and see whether it changes with the condition.
5. Do a wiggle test on the harness with the scan tool in graph mode.
6. Check continuity of each wire to the computer (under 1 ohm).
7. Check for ABS and traction control codes.

How to fix the P0856 code

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

Technical reference data

• The switch or sensor is powered from the computer. Check the values in the service manual.
• Reference values are only a guide, so look up the manufacturer data for your vehicle.
• Wire continuity should be under 1 ohm, and ground voltage drop under 0.1 V.

Brand-specific notes

The letter assignment and the location of the component change from one manufacturer to another, so check the manual for your vehicle before you start testing.

The traction control signal usually starts at the ABS module, so a P0856 code that shows up alongside ABS or traction control codes often points to the module side and not to the transmission.

Frequently asked questions about P0856