P0856 code definition
The P0856 trouble code detects an issue with the traction control input signal.
What the P0856 code means
The P0856 code is a standard OBD-II trouble code that notes a problem with the traction control input signal. The traction control input signal is responsible for sending data to the powertrain control module (PCM), and other control modules. The PCM and the other control modules use this data to determine the proper revolutions per minute (RPM) for the vehicle, as well as engine timing, fuel delivery, and many other factors. When the PCM and accompanying control modules do not get input signals from the traction control module, then the P0856 trouble code will be triggered.
What causes the P0856 code?
The P0856 trouble code is usually caused by a faulty ON/OFF switch in the traction control system. This usually is the result of spills or leakage damaging the control switch, or faulty, loose, or damaged wires, connectors, and other electrical components in the system. In addition to a defective switch, corrosion on the traction control sensor connector can cause the P0856 code to be detected. While it is rare, the traction control module may also be defective.
What are the symptoms of the P0856 code?
When the P0856 trouble code is detected, the check engine soon warning light will illuminate on the instrument cluster, and the traction control malfunction warning light will likely illuminate as well. In addition to these warnings, the traction control system is likely to stop working, which can result in a loss of grip (mainly on slippery or icy conditions). The transmission may also have a hard time shifting, or may not shift at all (and may enter limp-in mode), and the vehicle will likely experience reduced fuel economy.
How does a mechanic diagnose the P0856 code?
The P0856 code should be diagnosed using a standard OBD-II trouble code scanner. A trained mechanic will use the scanner to view the freeze frame data, and assess the code. The mechanic will also look for the presence of additional trouble codes. The codes should then be reset, and the vehicle should be restarted, to see if the code persists. If the code does not return, then it was likely triggered erroneously, or represents an intermittent issue.
If the P0856 trouble code returns, the mechanic should visually inspect the electrical components in the traction control system, to see if there are any loose, damaged, or corroded wires and connectors. All damaged components should be replaced. If the technician notices that the ON/OFF switch is damaged as well, it should be replaced.
Every time a component is replaced, the codes should be reset and the vehicle restarted before the mechanic proceeds. This will allow the mechanic to tell whether the issue has been resolved, or if there is more work to do.
Common mistakes when diagnosing the P0856 code
The most common mistake made when diagnosing the P0856 code is the failure to adhere to the OBD-II trouble code diagnosis protocol. All mechanics should follow the protocol properly, to ensure that both the diagnosis and the repair is not only properly done, but efficient as well.
It is common for the traction control module to be replaced when the only issue was a faulty ON/OFF switch or some damaged electrical components.
How serious is the P0856 code?
A vehicle with the P0856 trouble code present is still drivable. However, the vehicle’s traction control system will likely be disabled, meaning the car will not have good traction on wet or icy roads. Furthermore, unnecessary stress may be placed on the transmission, which can ultimately cause damage. Even though the car may be drivable, the P0856 code should be inspected and resolved as quickly as possible.
What repairs can fix the P0856 code?
Some possible repairs for the P0856 trouble code include:
- Replacement of wires and connectors in the traction control module
- Replacement of the ON/OFF switch in the traction control module
- Replacement of the traction control module itself
Additional comments for consideration regarding the P0856 code
The P0856 trouble code only applies to cars that have electronic traction control systems.
Need help with a P0856 code?
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This article originally appeared on YourMechanic.com as P0856 OBD-II Trouble Code: Traction Control Input Signal and was authored by Brady Klopfer.