P0703 Code: Brake Switch B Circuit

What the P0703 code means

The brake pedal switch does more than it seems. Besides turning on the brake lights, it tells the engine computer, the transmission and several driver assistance systems that the driver is braking.

With that signal, the transmission unlocks the torque converter, cruise control cancels at once, the shift lock lets you take the lever out of park and, on many vehicles, the starting system allows the engine to start.

The P0703 code is stored when that signal is missing, inconsistent or does not change when it should. Most vehicles have two contacts in the same switch, one normally open and one normally closed, precisely so the computer can detect inconsistencies. If both report the same thing, something does not add up.

The most frequent cause is not a failed part but an adjustment. The switch is mounted at a precise position relative to the pedal, and it only takes a few millimeters of shift, from a knock or from plain use, for it to stop switching properly.

It helps to see how many systems depend on such a simple component, because it explains why the symptoms are so scattered. The brake pedal signal is used by cruise control to cancel, by the transmission to unlock the torque converter, by the shift lock to let you leave park, by the starting system on vehicles that require the brake to be pressed, and on many models by the hill start assist. When it fails, the driver notices half a dozen apparently unrelated problems and tends to suspect a serious electrical fault, when the usual cause is a twenty dollar switch that is out of adjustment.

It is worth knowing why the switch has two contacts and not one. With only one, a cut wire and an unpressed pedal would produce the same reading, and the system could not tell them apart. With two contacts that work in opposite directions, one closed while the other is open, any inconsistent combination reveals a fault right away.

That design explains a useful diagnostic detail: it is not enough to check that the brake lights come on. They can work perfectly because they depend on one of the contacts, while the other one, the one that reports to the computer, is failing.

P0703 code symptoms

• Cruise control does not engage or cancels by itself.
• Check engine light on.
• The shift lever will not come out of park when you press the brake.
• Harsh shifts, or the torque converter does not unlock.
• Brake lights stay on all the time or do not come on.
• The engine may not start on vehicles that require pressing the brake.
• Slightly rough idle when braking.
• The brake lights work normally and the code still persists.

Likely causes of the P0703 code

1. Switch out of adjustment relative to the pedal. The most frequent cause.
2. Worn internal contacts in the switch.
3. Deteriorated or missing rubber stop on the pedal, which changes the rest position.
4. Loose connector or backed-out pins.
5. Wiring damaged under the dash.
6. Blown fuse in the circuit.
7. Switch installed with the wrong adjustment after a replacement.
8. Poor ground in the circuit.

How to diagnose the P0703 code step by step

1. Check whether the brake lights work and whether they go off completely when you release the pedal. This is the quickest check.
2. Watch the switch status in live data while you press and release the pedal. It should switch cleanly.
3. Inspect the rubber stop on the pedal. If it deteriorated or fell off, the pedal rests farther in and the switch ends up in the wrong position.
4. Verify the switch adjustment using the manufacturer procedure.
5. Measure the continuity of both contacts with the pedal pressed and released.
6. Inspect the connector for backed-out pins.
7. Check the fuse for the brake light circuit.
8. Inspect the wiring under the dash.
9. Test both contacts separately. The brake lights can work perfectly while the contact that reports to the computer is failing.

How to fix the P0703 code

• Adjust the switch following the manufacturer procedure. It is the most common repair and needs no parts.
• Replace the rubber pedal stop if it deteriorated or is missing.
• Replace the switch if its contacts do not switch cleanly.
• Clean the connector and replace any terminals with backed-out pins.
• Replace the fuse and find out why it blew.
• Repair damaged wiring under the dash.
• Restore the circuit ground.
• Do not assume the switch is good because the brake lights work. Test both contacts.

After the adjustment, check that the brake lights come on at the first touch of the pedal and go off completely when you release it, and confirm the P0703 code does not return.

Technical reference data

• Most switches have two contacts, one normally open and one normally closed, to detect inconsistencies.
• The engine computer, the transmission, cruise control and the shift lock all use the signal.
• The rubber stop on the pedal sets its rest position and affects the switch adjustment.
• The brake lights must go off completely when you release the pedal.
• The P0704 code refers to the clutch switch on manual transmission vehicles.
• The two contacts work in opposite directions, so any inconsistent combination reveals the fault right away.

Brand-specific notes

The rubber stop on the brake pedal is a part almost nobody looks at, and it explains quite a few cases. Over the years it hardens, cracks and finally falls off. Without it, the pedal rests a few millimeters farther in and the switch ends up in the wrong position. It costs very little and fixes the problem at the root.

Always check the brake lights. If they stay on all the time, besides the code you have a drain that can run the battery down in a few days.

Frequently asked questions about P0703