P1855
CAN Bus Software Version Control.
- Volkswagen
- Powertrain
- Manufacturer code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 1
- Defined byThe manufacturer
- 8
- AreaTransmission
- 55
- FaultThe specific fault in that area
What it means
On Volkswagen group vehicles, P1855 is logged as a CAN Bus Software Version Control fault. A control unit has checked the software or coding version reported by another unit on the CAN data bus and found it missing or not matching what it expects. The exact units involved and the version check used depend on the model and engine, so confirm them against the manufacturer's data for the vehicle.
Symptoms
- Engine management or gearbox warning light on
- Gearbox may go into a limp or emergency mode, or shift harshly
- Reduced performance or restricted engine output
- Other warning lights on the dash, such as ABS or ESP
- Communication errors or no response from one or more control units during a scan
- Usually no symptoms other than a stored fault, if the cause is a recent software or coding mismatch
Possible causes
- Control unit recently replaced, or fitted from another vehicle, with the wrong software version, coding or adaptationlikely
- Outdated or incomplete software on one control unit after a repair, update or interrupted flashlikely
- Incorrect or lost coding or adaptation data between the engine and gearbox control units (or other networked units)possible
- Low battery voltage or supply fault during programming or coding, leaving a unit in a corrupt statepossible
- CAN bus wiring, connector or terminal fault causing intermittent or corrupted messagespossible
- Internal fault in a control unit that stops it reporting its version correctlyrare
How to diagnose
- Read all stored fault codes in every control unit and note freeze frame data, looking for other CAN or communication faults alongside P1855.
- Find out whether any control unit has been replaced, reprogrammed or recoded recently, and whether the battery was disconnected or went flat.
- Check battery condition and charging voltage, as low voltage can cause communication and coding faults.
- Inspect the CAN wiring, connectors and terminals at the affected control units for damage, corrosion, water ingress or pushed-back pins.
- Check that every networked control unit responds in the diagnostic scan and compare their part numbers and software versions with the manufacturer's data for the model.
- Check the coding and adaptation values in the engine and gearbox control units against the specification for the vehicle.
- Check for applicable software updates or technical bulletins for the model and engine.
- If wiring and versions look correct, test the CAN bus signals and resistance to rule out a bus fault.
How to fix
- Fit a control unit with the correct part number and software version for the vehicle, then code and adapt it to the car.
- Update or reprogram the affected control unit with the current manufacturer-approved software, using a stable power supply.
- Correct the coding and adaptation values so all networked units match.
- Repair or replace damaged CAN wiring, connectors or terminals.
- Replace a weak battery before reprogramming or coding, then clear the fault and confirm it does not return.
- Replace a control unit that stays faulty after software and coding are correct.
Can I keep driving?
Medium severity
The car can often be driven carefully in the short term, but have it checked soon because gearbox or engine limp mode may occur. Avoid driving if the gearbox behaves erratically or there is a significant loss of power.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.