P1857

What it means

P1857 on a Volkswagen means the transmission control unit received an implausible or missing engine load signal from the engine control unit over the CAN data bus. The gearbox uses this signal to set shift points and clutch or line pressure, so without a valid value it falls back to a default strategy. The fault is usually a symptom of an engine-side or network problem, not a fault inside the gearbox itself.

Symptoms

• Gearbox warning light or gear selector warning on the dash
• Harsh, delayed or flared gear changes
• Transmission stuck in a single gear or in limp mode
• Engine management light on at the same time
• Poor or inconsistent acceleration
• Other communication faults stored in several control units

Possible causes

• Engine control unit fault or an engine sensor fault (for example airflow, manifold pressure or throttle position) giving a bad or unavailable load valuelikely
• CAN bus wiring fault, such as a chafed wire, a short or an open circuit, causing dropped or corrupted messageslikely
• Corroded, loose or water-damaged connector at the engine control unit or transmission control unitpossible
• Low or unstable battery or charging voltage causing temporary communication loss between control unitspossible
• Outdated software in the engine or transmission control unit with a known load signal issue (model and engine dependent)possible
• Internal fault in the transmission control unitrare

How to diagnose

1. Read all fault codes from the engine, transmission and gateway, and note which are current and which are stored, along with the freeze frame data
2. Deal with any engine fault codes first, particularly airflow, pressure, throttle and misfire codes, as they can affect the load signal
3. Check battery condition and charging voltage, and make sure the battery terminals and earth straps are clean and tight
4. Inspect the wiring and connectors at the engine control unit and transmission control unit for damage, corrosion, moisture and bent or pushed-back pins
5. Check the CAN bus wiring for continuity, shorts and correct resistance, following the wiring diagram for the model and engine
6. Using live data, compare the engine load value shown by the engine control unit with the value received by the transmission control unit while the engine is running
7. Check whether a software update or a technical bulletin applies to this model, engine and gearbox combination
8. Clear the codes and road test, then recheck to see whether the fault returns straight away or only under certain conditions

How to fix

• Repair any engine sensor or engine control unit faults that cause an invalid load signal
• Repair or replace damaged CAN bus wiring and restore proper insulation and routing
• Clean, repair or replace faulty connectors and terminals
• Restore battery and charging system health if the voltage is low or unstable
• Update the control unit software where an outdated version is a known cause
• Replace the transmission control unit, with correct coding and adaptation, only after the wiring, engine side and supply voltage have been proven good

Can I keep driving?

This code for other makes

• P1857OtherTransmission Transfer Case Contact Plate 'C' Short Circuit To Ground

Nearby codes

• P1856VolkswagenThrottle Pedal Position Sensor A Circuit Error Message From Engine Control
• P1858VolkswagenEngine Speed Input Circuit Error Message From Engine Control