OBD-II trouble code
P0572: Cruise Control / Brake Switch "A" Circuit Low
The vehicle believes your foot is on the brake permanently. The complaint that actually brings these cars in is often a battery that keeps going flat overnight rather than anything to do with cruise control.
Quick facts
- System
- Powertrain
- Category
- Cruise Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $20 – $480
- DIY difficulty
- Intermediate DIY
What does P0572 mean?
P0572 says the voltage on the brake switch "A" circuit sat below the range the module is willing to accept. The important interpretation step is that "low" describes electricity, not the pedal, and what a low reading means functionally depends entirely on how the circuit was designed. Some brake switches send battery voltage to the module when the pedal is pressed, in which case low means that supply has been lost. Others complete a path to ground when pressed, in which case low means the module is being told the pedal is down. You need the wiring diagram for your specific vehicle before this code means anything precise, and skipping that step is how people end up testing for the exact opposite of the fault they have.
On the far more common ground-switched arrangement, what this code amounts to is that the vehicle now believes the brake pedal is being held down all the time. That belief travels a lot further than most people expect, because the brake signal is one of the most widely consumed pieces of information on a modern vehicle. It is not just a lamp trigger.
Cruise control is the obvious casualty and the reason this code carries the cruise name — no control strategy will engage a system that is being told the driver is braking. But the transmission also uses it. Many strategies unlock the torque converter clutch the instant the brake is applied, so a permanently asserted brake signal can leave the converter unlocked at cruising speed. The driver does not see a warning for that. They see fuel economy fall, they feel the engine sitting a few hundred revolutions higher on the highway than it used to, and over time they get transmission fluid running hotter than it should. Stop-start systems are similarly affected: they take the brake signal as one of their conditions, so behaviour on that front becomes erratic. Hill-hold, where fitted, works from the same input.
Then there is the failure mode that actually gets these vehicles booked in, and it has nothing to do with driving. A brake circuit held low can keep brake lamps illuminated or keep a module from going to sleep after the vehicle is locked. The customer's complaint is a battery that is flat every second or third morning, and they arrive convinced they need a battery or an alternator. A stored P0572 in that context is a large clue, and the confirmation is quick: measure the current the vehicle draws while parked, then unplug the brake switch and measure again. If the draw collapses, the diagnosis is finished and no new battery is required.
Because a short to ground in the harness and a switch stuck closed produce an identical reading, the test that matters most is separating them, and it costs nothing: take the switch out of the circuit and look again. Everything downstream of that decision is either harness work or a switch, and there is no point doing one while suspecting the other.
Common causes
- Brake switch with a contact set stuck closed from internal wear
- Wiring chafed through to a bracket edge or stud in the under-dash loom
- Water intrusion into the switch connector from a wet footwell or a leaking heater core
- Circuit shorted to ground behind an aftermarket accessory splice
- Switch plunger held in by a jammed pedal, a floor mat or a distorted bracket
- Corroded terminal creating a resistive path to ground inside the connector
- Trailer wiring module faulting and pulling the brake circuit down
- Damaged module input on rare occasions, once switch and harness have been eliminated
Symptoms
- Battery flat after the vehicle has been parked overnight or for a couple of days
- Brake lights staying illuminated with the pedal released
- Cruise control refusing to engage at any speed
- Highway engine speed higher than usual and fuel economy noticeably worse
- Stop-start system behaving inconsistently or not activating
- Transmission running hot on longer journeys
- Shifter releasing from park without the pedal being touched
- Check engine light with no change in how the engine starts or runs
Diagnostic steps
- 1.Settle one thing before you measure anything: does your brake switch supply voltage to the module, or does it complete a path to ground? The wiring diagram will tell you in seconds and the answer inverts the meaning of the word "low". On a feed-side design a low reading means the supply has gone missing. On a ground-side design a low reading means the switch is behaving as though it is closed. Interpreting the code without knowing which one you have sends you looking for the opposite fault.
- 2.Unplug the brake switch and read the circuit again. This single test splits the whole diagnosis in two. If the reading stays low with the switch out of the circuit entirely, the switch is innocent and there is a fault in the harness or at the module. If the reading changes as soon as the connector comes off, the switch is what is holding it there.
- 3.With the switch disconnected, check that circuit against chassis ground for continuity. A dead short reads obviously. What you are really hunting is a partial short — a chafed insulation point that leaks to ground through a few hundred ohms, which is enough to hold the reading low and nowhere near obvious enough to find by looking.
- 4.Follow the pedal-switch leads up and along the under-dash loom and inspect every point where they cross a bracket edge, a mounting stud or a duct. This harness sits in the one part of the vehicle interior where sharp sheet metal, a moving pedal assembly and a footwell that occasionally gets wet all meet.
- 5.Measure parasitic draw if the customer has mentioned a flat battery, and do it before you replace anything. A brake circuit sitting low can hold lamps or a module awake with the vehicle locked, and confirming the draw disappears when the brake switch is unplugged proves the whole story in one measurement.
- 6.After the repair, check that everything the brake signal drives has come back: brake lamps extinguishing properly, the shifter releasing normally, and the torque converter and stop-start behaving as they should on a road test.
Repair cost
$20 – $480
A brake switch is $15 to $70 and half an hour of labour, so the straightforward version of this repair lands around $80 to $200. What pushes it up is harness work: tracing a short to ground under a dashboard means removing trim, sometimes the lower dash panel, and paying for the time it takes to find a chafe point that may be a few centimetres long. Budget $150 to $400 for that outcome. Diagnosis is $80 to $150 and pays for itself here more than usual, because the alternative story — a customer replacing a perfectly good battery and then a perfectly good alternator chasing an overnight drain — costs several times more than finding the switch.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with brake light switch replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.