What is the code for a bad transmission speed sensor?
In most vehicles, a bad transmission speed sensor shows up with diagnostic trouble codes such as P0715, P0716, P0720, or P0500. The exact code depends on the vehicle’s make and model, but these four are among the most commonly seen when the sensor or its circuit is impaired.
What the transmission speed sensor does
The transmission speed sensor (often referred to as a vehicle speed sensor, or VSS) provides a real-time speed signal to the engine control module (ECM) and, in many cars, to the transmission control module (TCM) and the vehicle’s speedometer. This data helps regulate shifting points, torque converter behavior, and idle speed. When the sensor or its wiring fails, you may notice rough shifting, flares, the transmission not shifting, or a faulty or inaccurate speedometer.
Common diagnostic trouble codes associated with a bad transmission speed sensor
Below are the codes most frequently linked to a faulty transmission speed sensor. Codes can vary by manufacturer and year, so always verify with your vehicle’s service manual or OEM data before acting.
- P0715 – Input Speed Sensor Circuit Malfunction
- P0716 – Input Speed Sensor Range/Performance
- P0720 – Output Speed Sensor Circuit Malfunction
- P0500 – Vehicle Speed Sensor Malfunction
These codes point to problems with the speed sensor itself, the wiring harness, or the related circuits that feed the data to the PCM/TCM. They do not pin down a single root cause, but they narrow the fault to the sensor path or data signal.
Understanding variations by manufacturer
Some brands may use additional or different codes for similar symptoms, and there can be minor wording differences across OEMs. For example, some systems may raise related “range/performance” or “circuit” codes for both input and output speed sensors. Always consult the specific code definitions from the vehicle’s manufacturer for precise interpretation.
Diagnostics and next steps
To determine whether the transmission speed sensor is truly at fault, follow a structured diagnostic approach. The steps below assume you have basic diagnostic tools and safe access to the vehicle.
Before starting the diagnostic list, review a practical, step-by-step plan to confirm the fault.
- Connect an OBD-II scanner and retrieve all stored codes and live data. Note which speed-related codes appear and monitor VSS readings while the vehicle is stationary and while moving, if safe to do so.
- Visually inspect the transmission sensor connector and wiring for signs of damage, corrosion, loose pins, or pinched harnesses. Repair or replace as needed.
- Test the sensor signal if you have the tools: measure the sensor’s output with the engine off and the key on, then observe live data readings or use a multimeter/oscilloscope to verify the signal changes with speed. Many VSS sensors are 0–5V or provide a frequency signal; refer to the service manual for correct testing procedures.
- Inspect the tone ring or reluctor wheel that the sensor reads. A damaged tone ring or misaligned sensor can mimic a bad sensor even if the electronic portion is fine.
- If the wiring and sensor appear intact but codes persist, try a known-good replacement sensor (or a donor vehicle test) to confirm whether the fault lies with the sensor itself.
- Clear codes after repairs and perform a road test to ensure the issue does not recur. Recheck for new codes or abnormal live data during the drive.
These diagnostic steps help distinguish a failed sensor from wiring issues, PCM faults, or mechanical transmission problems that can produce similar symptoms.
What to do next and when to seek help
If you see persistent speed-related codes, coughing or inconsistent shifting, or a mismatched speedometer reading, it’s prudent to have a professional diagnose the fault. Transmission systems are complex, and misdiagnosis can lead to unnecessary part replacements or more serious drivetrain issues. A qualified technician can verify sensor function, inspect the transmission’s internal circuits, and confirm whether a sensor replacement is sufficient or if wiring and PCM checks are also required.
Summary
A faulty transmission speed sensor often triggers codes such as P0715, P0716, P0720, or P0500. While these codes point to the sensor path or related circuitry, exact causes can vary by vehicle. A careful diagnostic process—covering code reading, wiring inspection, sensor testing, and, if needed, tone-ring verification—will help determine whether the sensor itself is defective or if the issue lies elsewhere in the signal path. If in doubt, consult a trained technician to avoid drivability and safety risks.
How serious is a P0500 code?
How serious is the P0500 code? The P0500 code is moderately serious because it indicates a problem with the Vehicle Speed Sensor (VSS), which provides critical data to the engine, transmission, speedometer, and cruise control systems.
What happens when a transmission speed sensor goes bad?
When a transmission speed sensor goes bad, it can cause harsh or incorrect shifting, and the cruise control may stop working. The "check engine" light will also likely turn on because the car's computer receives faulty speed data, which can affect other systems like the torque converter clutch and even the speedometer.
Common symptoms of a bad transmission speed sensor
- Poor shifting: The transmission may shift roughly, take longer to shift, or get stuck in a gear, leading to a jerky or uncomfortable ride.
- Cruise control failure: The cruise control will likely be disabled by the car's computer as a safety precaution, since it relies on accurate speed data to function correctly.
- Check Engine Light: The "check engine" light will turn on to alert you that the engine control unit (ECU) has detected a problem, such as a faulty speed sensor.
- Speedometer issues: The speedometer may display erratic readings, fluctuate unexpectedly, or stop working altogether.
- Other system failures: The anti-lock braking system (ABS) and traction control system (TCS) may also be affected, which can lead to reduced braking safety and traction.
- Reduced fuel economy: A malfunctioning sensor can prevent the torque converter clutch from engaging properly, causing slipping and reducing efficiency.
How serious is a P0720 code?
As we've mentioned, the P0720 code and its related codes can cause serious driveability issues, and you shouldn't risk prolonged driving of your car. You may cause further damage to your engine and transmission that will only lead to remarkably high repair bills if the transmission is compromised.
Will a transmission speed sensor throw a code?
The transmission's input speed sensor signals the TCM to enable it to shift the transmission to the proper gear needed for changes in speed. If the TCM is receiving an erratic signal, the P0715 code will be produced.
