P0859: Traction Control Input Signal High

What it means

The traction control input signal to the ECM reports a input signal high condition. Engine torque reduction during wheel slip cannot be commanded reliably.

At a glance

• Usually lights the check-engine light
• The engine or gearbox may go into limp mode

How it works

The traction-control module sends a torque-reduction request to the engine control module when wheel slip is detected. This code sets when the input signal is higher than expected, which can result from a wire shorted to battery voltage or a high module output.

Common causes

Most likely first.

1. Signal wire shorted to B+
2. TCS module output high

Symptoms

• Check engine or transmission warning light on
• Harsh, delayed, or missing shifts
• Possible limp mode (locked in one gear)
• Slipping or shuddering during gear changes

How to diagnose it

1. Record freeze-frame data and scan the engine and ABS/traction-control modules for related signal or communication codes.
2. Check live traction-control request data to determine whether the engine module sees a constant or unexpectedly high request.
3. Inspect the signal wiring and connectors for contact with a power feed, damaged insulation, or poor connections.
4. Test the input circuit for a short to battery voltage and check the traction-control module output according to the manufacturer's procedure.
5. If the circuit tests correctly, verify module powers, grounds, and communication before considering a module fault.

Common fixes

• Repair damaged wiring or a signal wire shorted to battery voltage.
• Clean and secure corroded or loose module connector pins.
• Correct a faulty module power or ground connection.
• Repair or replace a traction-control module with a confirmed high output.

Can I keep driving?

Traction control may request torque reduction incorrectly or fail to respond properly, and the transmission may enter a limited mode. Avoid slippery conditions and do not continue if the vehicle loses predictable acceleration or shifting.