Where does the vehicle speed signal come from?

On older cars, from a dedicated vehicle speed sensor mounted on the gearbox, driven by the output shaft. On virtually every car built in the last twenty years, there is no such sensor: road speed is calculated by the ABS module from the four wheel speed sensors and broadcast over the vehicle network for every other module to use. That is why a P0500 on a modern car so often turns out to be a wheel speed sensor or an ABS module fault rather than anything in the engine bay.

Will a broken speedometer fail an MOT?

Yes. The MOT includes a check that a speedometer is fitted, complete and working, and one that does not operate or cannot be read is a defect. With P0500 there is usually a second and separate failure as well, because the engine management lamp will be on and that is a major defect in its own right, recorded 371,859 times in the DVSA data behind this site.

Why are my ABS and engine lights both on with P0500?

Because they are almost certainly the same fault seen from two directions. If a wheel speed sensor or the ABS module has failed, the ABS module lights the ABS lamp and stops broadcasting a reliable road speed. The engine control unit then finds no usable speed signal and stores P0500 along with the engine lamp. Repairing the wheel speed sensor fault normally clears both, and it is worth reading the ABS codes before doing anything under the bonnet.

Can bigger wheels cause P0500?

Only in unusual cases, and it takes a large departure from standard. The control unit calculates road speed from wheel rotation using a stored rolling radius, so a substantially different tyre size gives a speed that disagrees with what the engine speed and gear ratio suggest. Modest changes stay inside the tolerance. If a car has been fitted with markedly different wheels, it is worth mentioning, but it is far more often a sensor or a wiring fault.

Powertrain code · standard

P0500Vehicle speed signal missing

Standard definition: Vehicle Speed Sensor "A" Malfunction

Last updated 27 July 2026

Quick answer

The engine control unit is not getting a usable road speed signal. On older cars that means a dedicated speed sensor on the gearbox; on most modern cars the speed comes from the ABS system, so the fault is often not in the engine bay at all.

How the car works this out

Road speed is used for far more than the speedometer: it feeds the gearbox strategy, cruise control, stability control, power steering assistance and the fuelling maps. The control unit checks that the speed signal is present and that it makes sense against engine speed and gear. P0500 is set when the signal is missing or implausible — for instance zero road speed while the engine is pulling in gear. Understanding where the signal comes from matters, because on almost every car built in the last twenty years it is calculated from the ABS wheel speed sensors and sent over the network, not generated by a separate sensor.

What you might notice

Speedometer reading zero or behaving erratically
Cruise control refusing to engage
Harsh, delayed or unpredictable automatic gearchanges
ABS and stability control warning lamps on as well
Power steering assistance feeling too light at speed on some cars
Odometer not counting up

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A faulty wheel speed sensor, on cars that derive road speed from the ABS systemCommon
How to check: Have all four wheel speed values compared on live data while the car is driven slowly. One reading zero or jumping about identifies the corner.
2A failed dedicated vehicle speed sensor on the gearbox, on older cars that have oneCommon
3A fault in the ABS module or a loss of communication with it, so the speed message is never broadcastPossible
How to check: Check whether ABS codes or a lost-communication code are stored alongside. If they are, fix those first — the speed code is a consequence, not a cause.
4Damaged wiring or a corroded connector, commonly where the loom runs to the gearbox or along a suspension armPossible
5A damaged tone ring or a driven gear stripped inside the gearbox on older mechanical sensorsPossible
6Non-standard wheel and tyre sizes far enough from standard to make the calculated speed implausibleLess common

What to do about it

1Find out first whether your car has a dedicated speed sensor or takes its speed from the ABS system. That single question determines whether this is a small job on the gearbox or an investigation of the braking system.
2Read the codes from the ABS module as well as the engine. A P0500 with ABS codes alongside is almost always the ABS system's fault, and repairing that clears both.
3Have all four wheel speeds compared on live data. It takes minutes and points straight at the corner responsible.
4Check the wiring at the gearbox or along the suspension where the loom flexes, particularly if the fault is intermittent.
5Mention any recent change of wheel or tyre size, and any recent work on the brakes or hubs.

Can you keep driving?

Only to a garage

It is not mechanically dangerous, but a car with no working speedometer is a legal problem as much as a practical one, and if the cause lies in the ABS system then the anti-lock braking and stability control will be disabled too. The brakes still work normally in ordinary use; what you lose is the assistance under emergency braking.

Will it fail an MOT?

This code can cost you the test in more ways than most. The engine management lamp is a major defect with 371,859 recorded failures. Separately, the MOT includes a check that a speedometer is fitted and working, so a dead speedo is a defect in its own right. And if the road speed signal is missing because the ABS system has failed, the brake system warning device showing a malfunction is a major defect too, with 85,196 recorded failures against wording that DVSA reuses for the anti-lock and electronic braking systems.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Where does the vehicle speed signal come from?: On older cars, from a dedicated vehicle speed sensor mounted on the gearbox, driven by the output shaft. On virtually every car built in the last twenty years, there is no such sensor: road speed is calculated by the ABS module from the four wheel speed sensors and broadcast over the vehicle network for every other module to use. That is why a P0500 on a modern car so often turns out to be a wheel speed sensor or an ABS module fault rather than anything in the engine bay.
Will a broken speedometer fail an MOT?: Yes. The MOT includes a check that a speedometer is fitted, complete and working, and one that does not operate or cannot be read is a defect. With P0500 there is usually a second and separate failure as well, because the engine management lamp will be on and that is a major defect in its own right, recorded 371,859 times in the DVSA data behind this site.
Why are my ABS and engine lights both on with P0500?: Because they are almost certainly the same fault seen from two directions. If a wheel speed sensor or the ABS module has failed, the ABS module lights the ABS lamp and stops broadcasting a reliable road speed. The engine control unit then finds no usable speed signal and stores P0500 along with the engine lamp. Repairing the wheel speed sensor fault normally clears both, and it is worth reading the ABS codes before doing anything under the bonnet.
Can bigger wheels cause P0500?: Only in unusual cases, and it takes a large departure from standard. The control unit calculates road speed from wheel rotation using a stored rolling radius, so a substantially different tyre size gives a speed that disagrees with what the engine speed and gear ratio suggest. Modest changes stay inside the tolerance. If a car has been fitted with markedly different wheels, it is worth mentioning, but it is far more often a sensor or a wiring fault.

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