P0502
Vehicle Speed Sensor Circuit Low Input
Get Your FREE AI Diagnostic Report
Enter your vehicle for a personalized diagnosis for code P0502
The P0502 code means the powertrain control module (PCM) is detecting a signal from the vehicle speed sensor (VSS) that is below the minimum expected voltage threshold. This is specifically a 'low input' code, meaning the electrical circuit is reading too low rather than being completely absent or too high.
The most obvious symptom is a speedometer that reads zero or drops to zero while driving. Your transmission may refuse to upshift because the PCM thinks the vehicle isn't moving fast enough, and cruise control will be disabled. Many vehicles will also illuminate ABS and traction control warning lights because those systems depend on accurate speed data.
The most common culprit is a wiring problem — a wire that has broken, shorted to ground, or developed high resistance due to corrosion. Start your diagnosis by inspecting the wiring harness from the VSS on the transmission all the way back to the PCM connector. Look for chafed insulation where the harness passes near hot or moving parts. If the wiring checks out, the sensor itself is the next most likely cause. Testing with a multimeter for proper resistance and using a scan tool to read live speed data can quickly narrow down the issue.
Severity
Symptoms
- •Speedometer drops to zero intermittently or stays at zero
- •Transmission may not shift out of lower gears
- •Cruise control is inoperative
- •ABS and traction control warning lights illuminate
- •Vehicle may enter limp mode in some models
Likely Causes
A broken wire or short to ground in the speed sensor circuit pulls the voltage below the PCM's minimum threshold, triggering the low-input code.
The sensor has failed in a way that produces little or no output voltage, staying below the expected signal level.
Moisture or road salt has corroded the connector pins, creating high resistance that drops the signal below the readable threshold.
A cracked or contaminated tone ring produces a weak magnetic signal that stays below the sensor's detection threshold.
The PCM's internal speed sensor input circuit has a fault that prevents it from reading normal voltage levels.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Repair or replace shorted/open wiring in the VSS circuit
- Replace the vehicle speed sensor
- Clean and secure the VSS connector and pins
- Inspect and replace the tone ring if damaged
Shop Parts
Videos
Causes and Fixes P0502 Code: Vehicle Speed Sensor A Circuit Low Input
HVAC Mechanic
How to Fix P0502 Code - Common Issues & Solutions ||
Auto Repair
The Dumbest way to “FIX” P0502 code on a Subaru
Truck Freak