Wiring fault in the traction control signal circuit, such as a short to voltage or chafed or damaged wires; Corroded, loose or damaged connector at the engine control module or ABS/traction control module; Fault with a wheel speed sensor or its circuit feeding the traction control system; Faulty ABS or traction control module sending an incorrect signal; Low battery voltage or charging system fault causing implausible signals; Fault in the engine or powertrain control module itself or its software; Faulty traction control switch or related input on models that use one.

P0859

Traction Control Input Signal High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
59: FaultThe specific fault in that area

What it means

P0859 is the generic code for a high input signal on the traction control circuit to the engine or powertrain control module. The control module monitors the communication or signal line between itself and the traction control or ABS module, and it set the code because the signal sat higher than expected, for example stuck high, shorted to voltage or showing an implausible value. The exact circuit and how the signal is carried (hardwired or over the data network) depends on the make and model.

Symptoms

Traction control or ABS warning light on the dashboard
Engine warning light (MIL) on
Traction control not working or switched off
Reduced or no engine torque intervention when the wheels slip
Possible rough running or hesitation under traction control activation
Other stored ABS or wheel speed sensor codes

Possible causes

Wiring fault in the traction control signal circuit, such as a short to voltage or chafed or damaged wireslikely
Corroded, loose or damaged connector at the engine control module or ABS/traction control modulelikely
Fault with a wheel speed sensor or its circuit feeding the traction control systempossible
Faulty ABS or traction control module sending an incorrect signalpossible
Low battery voltage or charging system fault causing implausible signalspossible
Fault in the engine or powertrain control module itself or its softwarerare
Faulty traction control switch or related input on models that use onerare

How to diagnose

Read all stored codes and freeze frame data, noting any ABS, wheel speed or network codes alongside P0859
Check the battery condition and charging voltage, as low voltage can set false signal faults
Visually inspect the wiring and connectors between the engine control module and the ABS/traction control module for damage, chafing, corrosion or water ingress
Check the traction control switch and its wiring if the model has one
Check wheel speed sensor live data for implausible or missing readings and compare all four wheels
Using the manufacturer wiring diagram, test the signal circuit for short to voltage, open circuit and correct reference values
Check for relevant technical service bulletins and whether software updates exist for the control modules on that model
Clear the code and road test to see whether it returns and under what conditions

How to fix

Repair or replace damaged wiring and renew faulty connectors or terminals
Clean and secure corroded or loose connector pins
Replace a faulty wheel speed sensor or repair its circuit if it is the source of the fault
Repair or replace the ABS/traction control module if testing proves it is at fault
Restore correct battery and charging system condition
Update or reprogram the control module software only if an outdated or faulty calibration is confirmed as the cause

Can I keep driving?

Medium severity

The car can usually be driven carefully, but traction control and possibly ABS may not work, so take extra care on wet or slippery roads. Get it diagnosed soon, and stop driving if the ABS or brake warning lights also come on or the car runs badly.

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