P0502: Vehicle Speed Sensor A Circuit Low

The vehicle speed signal A comes, depending on the maker, from a sensor on the gearbox, the transmission output speed, the wheel speeds via ABS/ESP or a CAN message; CARB explicitly names it as an input to be monitored. Despite its wording, all makers found check plausibility: the car is clearly moving (load, torque, overrun), yet the signal stays at zero. GM's 2018 tables use the transmission output speed: below 12 rpm (petrol) or 60 rpm (diesel) for 4.5 s, MIL on the first trip. Subaru (also in the Saab 9-2X) sets it below 1 km/h on overrun for over 4 s with the MIL at once; Saab's T5 after 4 s without signal above 1800 rpm. Electrical faults give the same picture.

Symptoms

• Check engine light on
• Speedometer dead or jumping
• Cruise control not working
• Automatic shifts harshly or wrongly

Most likely causes

• Speed or output speed sensor faulty
• Wire open or shorted to ground, connector corroded
• ABS module sends no speed signal
• Tone wheel damaged or sensor dirty

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the speed in live data while someone else drives
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Inspect the connector and wiring
4. Read the ABS codes as well
5. Replace the sensor if there is no signal

Good to know

• Repair difficulty: medium
• It can trigger limp mode.
• It affects emissions.

What each character of P0502 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 5: Speed and idle
• 02: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0502?

Drive with caution: the speedometer may be wrong and driver assistance features may fail.

What causes code P0502?

The most common causes are: speed or output speed sensor faulty; wire open or shorted to ground, connector corroded; aBS module sends no speed signal.

Related codes

• P0500Vehicle Speed Sensor A Circuit
• P0501Vehicle Speed Sensor A Circuit Range/Performance
• P0503Vehicle Speed Sensor A Circuit Intermittent/Erratic/High
• P0722Output Shaft Speed Sensor Circuit No Signal
• P0720Output Shaft Speed Sensor Circuit

Sources

• gsi.ext.gm.com: 18OBDG03A Engine Diagnostics
• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
• saabwisonline.com: dtc p0502 vehicle speed sensor circuit low input
• saabwisonline.com: p0500p0501 p0502

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.