Cruise control gone: codes P0500, P0501, P0504, P0571
Cruise control stopped engaging, the speedometer froze or jumps, and the gearbox started shifting at the wrong moments. All of it is one sensor — or one penny switch under the brake pedal.
If the symptoms arrived together — cruise, speedometer and shifting — look for one place, not three. Only one source in the car knows the speed, and everyone else feeds from it.
Where the car gets its speed from
The speedometer is not a separate instrument. It draws the same number the control unit receives from the sensor on the gearbox. That same number goes to the cruise control, to the ABS, to the shift decision and to the fuel consumption calculation.
So vehicle speed in the app is not «another speedometer» but a look at the original source. Worth knowing too: the dashboard speedometer usually overreads on purpose by a few per cent, so a disagreement with the app is not in itself a fault.
Speed codes: P0500, P0501, P0502
P0500 — speed sensor malfunction. There is no signal.
P0501 — reading out of range. The unit sees that the speed is not changing although the revs are rising and a gear is engaged. That is what a corroded connector or a stripped drive gear looks like.
P0502 — low signal. An open circuit or a short to ground.
The check takes one drive: open speed in the app and cover a hundred metres. The number sitting at zero while moving means the sensor. Jumping and dropping out means a contact. Two or three per cent off the speedometer is normal.
⚠️ On many cars the speed sensor and the transmission output speed sensor are the same device. That is why transmission codes often sit next to P0500: read the transmission walkthrough before repairing two assemblies separately.
The brake light switch: P0504 and P0571
Here is the cause almost nobody looks for when cruise control stops engaging.
Under the brake pedal sits a switch — the one that lights the brake lamps. It has two contacts working against each other: one closes when the pedal is pressed, the other opens at the same moment. That is how the car checks itself.
P0504 — the signals do not correlate: the contacts disagree. P0571 — circuit malfunction.
The consequences are out of proportion to the price of the part:
- cruise control switches off entirely and will not engage — the unit is not allowed to hold a speed if it is unsure it can see the brake being pressed. The cruise lamp may not even light up;
- on many cars Drive will not engage without the brake pressed — which looks like a gearbox fault;
- the brake lights stay on permanently, or never come on.
It takes a minute to check, with a helper and no instruments: have someone press the brake and look at the lamps. Lit with the pedal released, or going out with a delay, means the switch. The part is cheap, is adjusted by a screw, and often simply gets knocked out of position after a repair.
What to do
- Compare the speed in the app with the speedometer on a level road.
- Look at the brake lights from outside — that answers the P0504 question.
- Inspect the sensor connector on the gearbox: it is underneath, in the dirt, and suffers first.
- Check whether U codes are sitting nearby — lost communication. If the cluster is not receiving data over the bus, the speedometer will freeze with a healthy sensor.
- Remember the ABS: on some cars speed is derived from its sensors, and then the ABS lamp is on as well.
