Error Code P0859 is defined as Traction Control Input Signal High. Error Code P0859 is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made from 1996 up to the present. This code only applies to vehicles equipped with an electronic traction control system. Specifications on the definition, troubleshooting, and repairs vary between make, model, and powertrain configuration.

The PCM (powertrain control module) and a set of modules receive data from the traction control input signal. The PCM (and other modules) uses this data to determine the proper RPM, engine timing, fuel delivery of the engine, and other various functions. When the PCM or other related modules doesn’t receive the input from the traction control module, it will register the error code. The traction control light may be activated in some vehicle models and may put the transmission into Limp-in mode.

Common Symptoms

This code’s most common symptom is a failed traction control system, which is pretty common in slippery driving conditions. Other common symptoms include:

  • Gears unable to shift
  • Increase in fuel consumption

Possible Causes

A faulty traction control On/Off switch is the most likely culprit for this code. Other common causes include:

  • Damaged electrical components and disconnected wirings
  • Corroded connectors
  • Liquid spilled into the traction control switch

How to Check

To diagnose this code, you will need an advanced scanned, DVOM (digital volt/ohmmeter) specialized scanner to test the Controller Area Network (CAN) bus harness, wiring diagram of the CAN Bus system.

Inspect all connectors and wirings, look for signs of damage such as burnt wires and corroded connectors.

Download the trouble codes or freeze frame data. Install the memory-saving device and then use the CAN Bus scanned to test the CAN Bus harness pins. This can be a time-consuming process.

Then, check the CAN Bus circuits, connectors, control modules, and fuses using the DVOM.

When checking these parts, along with the wirings and connectors, make sure to test with the reference voltage and continuity with battery ground.

Then, check for continuity and ground at the traction control On/Off switch using the DVOM.

Make sure you clear and retest the system to see if the problem returns.

How to Fix

  • Repair or replacement of defective or faulty connectors, wires, and other components
  • Replacement of the damaged, full CAN Bus harness (it’s often easier to replace the whole thing than repairing individual wiring)
  • Replacement of faulty control modules
  • Replacement of blown or faulty fuses
  • Replacement of damaged traction control On/Off switch

The most common mistake in diagnosing this code is assuming that the traction control module is at fault when it’s the faulty switch, connector, or circuit. Thus, make sure you check for these things before replacing the traction control module.

This code usually doesn’t affect the vehicle. However, it may cause a traction issue in certain conditions. Also, in some instances, it leads to shifting problems, decreasing the vehicle’s fuel efficiency.