P0502 Code: Vehicle Speed Sensor Circuit Low Input

What the P0502 code means

The speed sensor tells the computer how fast the vehicle is going. It feeds the speedometer, the cruise control, the automatic transmission shifts and the idle control. On many modern vehicles the speed comes from the ABS wheel speed sensors over the CAN network.

The sensor generates pulses whose frequency depends on the speed or the quantity being measured. The P0502 code is a low signal, which is usually caused by the signal wire shorted to ground or by a missing reference.

Without a reliable speed, the automatic transmission can fall back to a limp gear, the speedometer stops and the cruise control shuts off. If the vehicle uses the ABS sensors, check the codes in that module as well, because a damaged wheel sensor affects everything. Look up the sensor guide to see how it is tested.

To confirm it, unplug the component with the key on and watch what happens to the reading or the code. If the value jumps to the opposite extreme, the wiring is fine and the component is the one pulling the signal to ground. If it does not change, look for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0502 code symptoms

• Check engine light on.
• Wrong speedometer reading and incorrect shifts on the automatic transmission.
• Speedometer stopped or erratic.
• Harsh shifts, or the transmission does not shift.
• Cruise control that does not work.

Likely causes of the P0502 code

1. Signal wire shorted to ground from the harness rubbing against the engine, the chassis or a bracket.
2. Power reference shorted. If several sensors fail at once, suspect the shared reference.
3. Damaged sensor with an internal short.
4. Connector with water or corrosion, which creates a path to ground.

How to diagnose the P0502 code step by step

1. Read the sensor value on the scan tool and compare it with what you expect.
2. Inspect the connector for corrosion, water or loose pins.
3. Check power and ground with the key on: correct reference and less than 0.1 V of voltage drop on the ground.
4. Measure the signal with the connector plugged in and see whether it changes with conditions.
5. Do a wiggle test on the wiring with the scan tool in graph mode.
6. Check the continuity of each wire to the computer (less than 1 ohm).
7. Compare the scan tool speed with a GPS.
8. Check the ABS codes if the speed comes from the wheel sensors.

How to fix the P0502 code

• Repair the connector or the wiring.
• Replace the sensor if the tests confirm it is bad.
• Correct the real condition if there is one.
• Clear the codes and verify with a road test.

Technical reference data

• Inductive sensor (2 wires) or Hall effect sensor (3 wires).
• Frequency proportional to speed.
• Approximate values only: check the manufacturer data for your vehicle.

Brand-specific notes

Where the speed signal comes from depends on the vehicle. On older trucks it comes from a sensor on the transmission or the differential.

P0502 on Toyota

On many modern Toyotas the speed comes from the ABS.

P0502 on Honda

On many modern Hondas the speed comes from the ABS.

P0502 on GM

On many modern GM vehicles the speed comes from the ABS.

Frequently asked questions about P0502