P0859 Code: Traction Control Input Signal High

What the P0859 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 P0859 code is set when that signal sits above its maximum. A high signal usually comes from an open circuit or from an interrupted ground.

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

To confirm the diagnosis, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be under 1 ohm, and an infinite reading means the wire is open. If the wiring is good, measure the component itself: an infinite resistance confirms it is open inside.

P0859 code symptoms

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

Likely causes of the P0859 code

1. Loose or broken connector. This is the most frequent cause.
2. Signal wire or ground wire cut, so the signal climbs up to the reference voltage.
3. Sensor open inside, with infinite resistance.
4. Short to 5 V or 12 V from rubbing against another wire in the harness.

How to diagnose the P0859 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 P0859 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 P0859 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 P0859