P1520
Cruise Control Switch S40.
- Mercedes-Benz
- Powertrain
- Manufacturer code
- Low severity
How this code is built
- P
- SystemPowertrain
- 1
- Defined byThe manufacturer
- 5
- AreaVehicle speed, idle control and auxiliary inputs
- 20
- FaultThe specific fault in that area
What it means
On Mercedes-Benz, P1520 relates to the cruise control switch, referred to as S40 in the manufacturer's documentation. The control unit has seen an implausible or faulty signal from the switch circuit, such as a stuck, open or shorted input, or a signal that does not match the expected operating pattern. The exact switch design and which control unit monitors it depend on the model and year.
Symptoms
- Cruise control will not set, resume or cancel
- Cruise control cuts out unexpectedly
- Cruise control warning or fault message in the instrument cluster
- Stored fault with the engine management or cruise control light on in some models
- Steering wheel or stalk switch feels unresponsive or sticky
Possible causes
- Worn, dirty or faulty cruise control switch or stalk (S40)likely
- Damaged wiring, corroded terminals or poor connector contact in the switch circuitlikely
- Faulty clock spring or steering column wiring on models where the switch signal passes through itpossible
- Brake light switch or pedal switch fault causing an implausible cruise cancel signalpossible
- Low or unstable battery voltage or earth fault affecting switch signalspossible
- Faulty control unit input or software issue (model dependent)rare
How to diagnose
- Read all stored codes and freeze frame data, then note whether the fault is current or stored and check for other related codes
- Operate the cruise control switch through all positions and check for a stiff, sticky or inconsistent action
- Visually inspect the switch, stalk connector and wiring for damage, moisture or corrosion
- Check the brake switch operation and brake light function, as these can affect cruise logic
- Check battery condition, charging voltage and main earth connections
- Use the live data or actual values to see whether the switch status changes cleanly with each button press, following the model-specific procedure
- Test the switch circuit for continuity, resistance and shorts to earth or supply against the manufacturer's wiring diagram
- Check the clock spring and column wiring if the signal is lost only when the steering wheel is turned
How to fix
- Replace the faulty cruise control switch or stalk
- Repair or replace damaged wiring and clean or replace corroded connector terminals
- Replace the clock spring or steering column wiring section if found faulty
- Replace a faulty brake light or pedal switch
- Correct battery or earth problems, then clear the code and confirm it does not return
- Replace or update the control unit only if a fault is confirmed and the manufacturer specifies it
Can I keep driving?
Low severity
The car is normally safe to drive, but cruise control may not work. Book it in for diagnosis, and take care if the brake lights or other systems also show faults.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.
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