MotorCodex Español

P1520 Toyota — Stop Lamp Switch Signal Malfunction

Moderate

Quick answer

P1520 means the stop lamp (brake light) switch signal reaching the engine computer doesn’t make sense — Toyota/Lexus monitors the brake switch because cruise control, stability control and the shift interlock all depend on it. A worn or misadjusted brake switch, a blown STOP fuse, or wiring are the usual causes; the switch itself is a cheap part.

What it means

The brake pedal switch does far more than light the brake lamps on a Toyota: the engine computer uses its signal for cruise control cancellation, the transmission shift interlock, and as a cross-check input for stability and traction systems. To catch a lying switch, Toyota uses two signal circuits from it and watches that they agree — pedal pressed and released should flip both, in opposite directions, every time. P1520 sets when those signals disagree or one goes missing.

The component itself is humble: a plunger switch on a bracket above the brake pedal, clicked thousands of times a year. Wear, internal corrosion, or a bracket adjustment that drifted (often after pedal-area work, or when the pedal stopper pad crumbles with age) all produce implausible signals. The STOP fuse and the wiring complete the short suspect list.

It’s worth diagnosing promptly for a non-engine reason: the same faults that set this code can leave your brake lights stuck off — invisible braking is a rear-end collision waiting to happen — or stuck on, which drains the battery and confuses drivers behind you.

P1520 symptoms: what you'll notice

  • Brake lights stuck on, stuck off, or flickering — have a helper confirm while you press the pedal.
  • Cruise control refusing to engage or dropping out — it requires a trustworthy brake signal.
  • The shifter refusing to move out of Park (the interlock needs the brake signal), or doing so inconsistently.
  • A dead battery overnight, when stuck-on brake lights are the cause.

Common causes

Ordered from most to least likely.

  1. 1.

    Worn or internally failed brake light switch

    A heavily used, inexpensive part — the most common ending.

  2. 2.

    Misadjusted switch or crumbled pedal stopper pad

    The little pad the switch plunger rests on disintegrates with age on many Toyotas — a famous cause of stuck-on brake lights.

  3. 3.

    Blown STOP fuse

    Takes one of the signal circuits down — free to check in the fuse box.

  4. 4.

    Wiring or connector damage at the pedal bracket

    Driver-footwell wiring suffers kicks, floor mats and the occasional water leak.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Watch the brake lights first

    Have a helper confirm: lights on with the pedal pressed, off when released, no flicker. Stuck on usually means the stopper pad or adjustment; stuck off points at the switch, fuse or wiring.

  2. 2 Check the STOP fuse

    Find it in the fuse box (lid diagram or manual) and inspect or test it. If it’s blown, also ask why — a chafed wire or trailer-harness fault may be the real story.

  3. 3 Inspect the switch and stopper pad at the pedal

    Look up above the brake pedal with a flashlight: the switch plunger should rest on an intact pad and travel with the pedal. Crumbs of plastic on the floor mat under the pedal are the disintegrated pad announcing itself.

  4. 4 Test the switch’s two circuits

    Unplug the switch and verify with a multimeter that its two circuits change state oppositely as you press the plunger. A switch that doesn’t flip both, cleanly, every time, is condemned — and it’s one of the cheaper parts on the car.

  5. 5 Adjust or replace, then confirm

    Fit or adjust the switch so the brake lights come on with slight pedal travel and go fully off at rest, clear the code, and confirm cruise control and shift interlock behave again.

Parts & tools you may need

Disclosure: some links are affiliate links (including the Amazon Associates program). If you buy through them, we may earn a commission at no extra cost to you.

Related codes

Frequently asked questions

What does code P1520 mean?
P1520 means the stop lamp (brake light) switch signal reaching the engine computer doesn’t make sense — Toyota/Lexus monitors the brake switch because cruise control, stability control and the shift interlock all depend on it. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.
Can I drive with P1520?
The engine runs fine, but check your brake lights before deciding: driving with no brake lights is genuinely dangerous (and ticketable), and stuck-on lights will flatten the battery and mislead drivers behind you. If the lights work, schedule the fix; if they don’t, fix it now.
Why does the engine computer care about the brake pedal?
Because several systems route through that signal: cruise control must cancel instantly on braking, the shifter’s interlock releases only with the brake pressed, and stability logic cross-checks braking against wheel speeds. A lying brake switch quietly degrades all of them, which is why it gets its own code.
My brake lights stay on all the time. Same problem?
Very likely — on many Toyotas the plunger of the brake switch rests on a small plastic stopper pad that crumbles with age; when it falls apart the switch thinks the pedal is always pressed. The replacement pad costs little and snaps in place in minutes.
Cruise control stopped working at the same time. Coincidence?
No — it’s the same fault wearing two hats. Cruise control refuses to engage without a plausible brake signal, by design. Fix the switch circuit and the cruise control returns with it.
Ask Codi