P0858: Traction Control Input Signal Low

What it means

The traction control input signal to the ECM reports a input signal low 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 an input to the engine control module to request torque reduction during wheel slip. This code sets when that input signal stays below its expected range, often because the wire is grounded or the traction-control module output is low.

Common causes

Most likely first.

1. Signal wire shorted to ground
2. TCS module output low

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. Save freeze-frame data and check ABS/traction-control and engine-module codes for clues about the low input.
2. Watch live traction-control request data to see whether the engine module reads a low or inactive signal when traction control is active.
3. Inspect the signal wire and connectors between the traction-control and engine modules for a short to ground, corrosion, or damage.
4. Test the signal circuit for a ground short and verify the traction-control module output using the manufacturer's procedure.
5. If the wire and module output test correctly, check module powers, grounds, and network communication before considering module replacement.

Common fixes

• Clean and secure a poor signal-circuit connector.
• Repair a signal wire shorted to ground.
• Restore a faulty traction-control module power or ground connection.
• Repair or replace the traction-control module if its output remains low with a verified circuit.

Can I keep driving?

Traction control torque reduction may not work as intended, and the vehicle may enter a transmission protection mode. Avoid slippery roads and stop driving if the transmission slips or cannot shift predictably.