Faulty vehicle speed sensor or gearbox output speed sensor; Damaged, chafed or corroded wiring, or an open or short to earth in the speed signal circuit; Poor connection at the sensor or control unit connector (corrosion, loose pin, water ingress); Faulty wheel speed sensor or damaged tone ring, where the speed signal comes from the ABS module; Metallic debris on the sensor tip or a damaged reluctor wheel or drive gear in the gearbox; Fault in the ABS module, instrument cluster or gearbox control unit supplying the speed signal; Fault in the engine control unit itself.

P0502

Vehicle Speed Sensor A Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
02: FaultThe specific fault in that area

What it means

P0502 is the generic SAE J2012 code for Vehicle Speed Sensor A Circuit Low. The engine control unit has seen the vehicle speed signal voltage or frequency below the expected range, or no signal at all, while other inputs such as engine speed and load suggest the car should be moving. The exact signal source (a gearbox output sensor, wheel speed sensor via the ABS module, or a CAN message) depends on the model.

Symptoms

Speedometer reads zero, drops out or behaves erratically
Malfunction indicator lamp on
Cruise control not working or cutting out
Harsh, delayed or incorrect gear changes on automatics
Poor or unstable idle when stopping, or stalling at low speed
ABS or traction control warning lamp on at the same time

Possible causes

Faulty vehicle speed sensor or gearbox output speed sensorlikely
Damaged, chafed or corroded wiring, or an open or short to earth in the speed signal circuitlikely
Poor connection at the sensor or control unit connector (corrosion, loose pin, water ingress)likely
Faulty wheel speed sensor or damaged tone ring, where the speed signal comes from the ABS modulepossible
Metallic debris on the sensor tip or a damaged reluctor wheel or drive gear in the gearboxpossible
Fault in the ABS module, instrument cluster or gearbox control unit supplying the speed signalrare
Fault in the engine control unit itselfrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting speed, engine speed and load when the code set, and check for ABS or gearbox codes
Compare the scan tool vehicle speed reading with the speedometer and a known road speed on a test drive
Inspect the speed sensor, wiring and connectors for damage, oil contamination, corrosion and loose pins
Check the sensor tip and tone ring or reluctor for debris, damage or missing teeth
Check the sensor supply and signal circuit for voltage, continuity and shorts to earth using the manufacturer wiring diagram
Check sensor resistance or output signal with a multimeter or oscilloscope while turning the wheel or output shaft, as per model-specific data
Check the signal path to the engine control unit, including the CAN data from the ABS module or cluster, for the specific model
Clear the code and road test to confirm whether it returns

How to fix

Replace the faulty vehicle speed sensor or gearbox output speed sensor
Repair or replace damaged wiring and connectors in the signal circuit
Clean or replace corroded terminals and reseal connectors against water ingress
Replace the faulty wheel speed sensor or damaged tone ring
Clean off debris, or repair the gearbox if the reluctor or drive gear is damaged
Repair or replace the faulty control unit supplying the signal, only once all other causes are ruled out

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but the lack of a speed signal can affect gear shifting, cruise control and idle control, and may involve ABS faults. Get it diagnosed soon, and limit driving if gear changes are harsh or the car stalls.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.