P0858 code definition
When the powertrain control module (PCM) detects a problem with the traction control input signal circuit, a P0858 trouble code is stored. The traction control input signal circuit delivers data to the PCM via a signal, which the PCM then uses to calculate how much the engine revolutions per minute (RPMs) are needed, engine timing, and how much fuel is needed for current vehicle traction. The P0858 trouble code only applies to vehicles that have an electronic traction control system. Some related trouble codes include P0856, P0857, P0859, and P0860.
What the P0858 code means
When the PCM, or a related module, doesn't receive input from the traction control module, a P0858 trouble code is stored and the check engine light illuminates. In some models, the traction control light might also illuminate and the transmission will be placed in Limp-in mode.
What causes the P0858 code?
A defective traction control ON/OFF switch is the most likely culprit behind the storage of a P0858 trouble code. Other causes can include damaged wiring or electrical connectors, corrosion in the sensor connector, and liquids spilled into the traction control switch.
What are the symptoms of the P0858 code?
Symptoms of a P0858 trouble code include the traction control system failing to operate, which is most obvious in slippery driving conditions. In addition to the warning lights, drivers can encounter problems when shifting gears and an increase in overall fuel usage while driving.
How does a mechanic diagnose the P0858 code?
To properly diagnose a P0858 trouble code, a mechanic needs an advanced scanner, digital volt/ohmmeter, a specialized scanner for testing the Controller Area Network (CAN) Bus harness, and a CAN Bus system wiring diagram. In addition, the mechanic should perform the following when diagnosing:
- Inspect the wiring, connectors, and components. In particular, look for faulty, corroded, or damaged items during inspection.
- Download any stored trouble codes or freeze frame data.
- Install a memory-saving device and test the pins of the CAN Bus harness for wiring for fault using a specialized CAN Bus scanner. The mechanic should keep in mind that this process is time consuming and cost prohibitive.
- Using a digital volt/ohmmeter, check the CAN Bus circuits, control modules, connectors, and fuses for fault.
- When checking these components, connectors, and wiring, make sure to test for reference voltage and continuity with the battery ground.
- Using the volt/ohmmeter, check the continuity and ground at the traction control ON/OFF switch.
- Clear and retest the system to see if the problem returns.
Common mistakes when diagnosing the P0858 code
Mechanics most commonly cite the traction control module as being at fault, when in fact the problem lies with a faulty switch, circuit, or connector. Subsequent replacement of the traction control module does not fix the problem causing the P0858 trouble code and the code returns.
How serious is the P0858 code?
While a P0858 trouble code usually doesn't affect the operation of a vehicle with the code, it can lead to issues with gaining proper traction in slippery conditions. In addition, transmission issues in the form of shifting problems are commonly experienced with this code, as well as a decrease in the overall fuel efficiency of the vehicle.
What repairs can fix the P0858 code?
Fixing a P0858 trouble code requires the mechanic to complete the following repairs:
- Replace any defective or faulty wiring, connectors, or components.
- If necessary, replace the full CAN Bus harness, as it is often easier to replace the whole thing as opposed to individual wiring.
- Replace any faulty control modules.
- Replace any blown or faulty fuses.
- Replace the traction control ON/OFF switch if it has gone bad.
Need help with a P0858 code?
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This article originally appeared on YourMechanic.com as P0858 OBD-II Trouble Code: Traction Control Input Signal Low and was authored by Cheryl Knight.