Why is my Chevy security light flashing and the car won't start?

On older GM vehicles this is the Passlock anti-theft system detecting a mismatch and disabling the start as a security measure. It is commonly triggered by a worn ignition lock cylinder, a worn key, or a failing Passlock sensor. GM's documented thirty minute relearn procedure requires no tools and resolves many of these cases at no cost, so it is worth attempting before paying for any repair.

What is the GM 30-minute security relearn procedure?

Three cycles of the following: attempt to start the engine, release the key back to the ON position rather than OFF, wait approximately ten minutes until the security light stops flashing, then turn the ignition off for about twenty seconds. After the third cycle the ignition must be turned fully off before the final start attempt, because the vehicle only learns the new code on an ignition transition from OFF to CRANK.

I completed the relearn and it did not work. What now?

First confirm the ignition was turned fully off before the final start attempt, as that single missed step causes most failed attempts. If the procedure was performed correctly and the vehicle still will not start, a component has likely failed rather than simply lost synchronization. Common causes include a worn ignition lock cylinder, a failed Passlock sensor, damaged wiring between the cylinder and the module, or a module requiring programming.

Is a flashing security light different from a solid one?

Yes. A flashing or blinking security light is the classic Passlock lockout signature and is the condition the relearn procedure is designed to address. A steady solid light accompanied by a no start condition is less likely to clear through a relearn and more often indicates a wiring fault, a failed sensor, or a module issue requiring diagnosis.

Should I bypass the GM security system with a resistor?

We advise against it. A bypass permanently disables the vehicle's anti-theft system, which can affect how an insurer views a future theft claim and is a matter requiring disclosure when selling the vehicle. Improvised wiring at the ignition also has its own failure modes, and owners frequently report bypasses failing again after a year or two. The underlying fault is usually a worn cylinder, failed sensor, or key that no longer reads correctly, all of which are repairable at the vehicle.

My newer Chevrolet says Key Fob Not Detected. Is that the same problem?

No, this is a different system and the Passlock relearn procedure does not apply. On push button start vehicles, first use the backup transmitter pocket, typically a specific cup holder or marked area in the center console, which allows the vehicle to read a fob with a completely dead battery. Then replace the fob battery, commonly a CR2032. A weakening battery frequently still performs remote start at distance while failing the short range proximity handshake, which is why strong remote start does not indicate a healthy battery.

Why does remote start work when the car cannot detect the fob?

Because the two functions use different communication methods. Remote start is a long range one way transmission, while proximity start requires a short range two way handshake between the fob and the vehicle that consumes more power. A fading battery typically fails the handshake before it fails remote start, which makes the assumption that working remote start proves a good battery misleading.

Chevy Security Light Flashing & Won't Start? The Full GM Guide

Start Here

First: Which GM Are You Dealing With?

Almost every piece of bad advice online comes from applying old-GM fixes to new-GM problems, or the reverse. The systems have almost nothing in common.

Roughly Through the Early 2010s

Passlock / VATS Era

→You turn a physical key in an ignition cylinder

→Dash shows a SECURITY light — flashing or solid

→Often no crank at all, or starts and immediately dies

→Security lives in a resistor pellet and sensor at the ignition lock cylinder

→There's a free relearn procedure that resolves many of these — see below

Mid-2010s and Newer

Keyless Access / Push-Button Era

→Push-button start, fob stays in your pocket

→Dash shows "Key Fob Not Detected" or "Place Key in Transmitter Pocket"

→Remote start may work while proximity start doesn't

→Usually a fob battery or antenna reception issue, not the ignition

→Different fixes entirely — the relearn procedure does nothing here

Older GM — Read the Light

Flashing or Solid?
It Genuinely Matters.

Most people just say "the security light is on." But how it's on tells you something, and it's worth noting before you call anyone.

✨

Flashing / Blinking

The classic Passlock lockout signature. The system sensed a mismatch and disabled starting. This is the one the relearn procedure is designed for — worth trying before spending anything.

🔴

Solid / Steady On

A steady light with a no-start is less likely to clear with a relearn and more often points to a wiring fault, a failed sensor, or a module issue. Owners frequently report the 30-minute procedure never completing on these.

⏱️

Comes On While Driving

Per GM's own owner documentation: if SECURITY appears while the engine runs, you can usually restart after shutting off — but the system isn't protecting the vehicle and needs service.

The Free Fix Worth Trying First

The 30-Minute Passlock Relearn

This is GM's own documented procedure. It requires no tools, no scanner, and no money — and it resolves a meaningful share of flashing-light no-starts. It's also the procedure people most often get wrong by one step.

✓ Before You Start

Make sure your battery is strong.

This procedure keeps the ignition in ON for thirty minutes total with the engine off, which draws steadily on the battery. A marginal battery can fail partway through and waste the whole attempt — owners often report this. If your battery is old or the car has been sitting, put a charger on it first. Also: close all the doors, and make sure the shifter is fully in Park.

1

Turn the key to START, attempt to start

The engine won't start — that's expected. You're triggering the system to enter the relearn state.

2

Release the key back to ON — not OFF

Leave it in the ON / RUN position. Do not turn it off. The dash lights should be on with the engine not running.

3

Wait about 10 minutes, watching the SECURITY light

After roughly ten minutes the light should stop flashing or turn off. Give it a little longer than exactly ten — many owners report needing eleven or twelve. Don't touch anything while waiting.

4

Turn the ignition OFF and wait about 20 seconds

A brief off period between cycles. Five to twenty seconds is what different GM documentation specifies; twenty is safe.

5

Repeat the whole thing two more times

Three complete cycles, about thirty minutes total. Yes, it's tedious. It's also free.

6

⚠ The step people miss

Turn the ignition fully OFF before the final start attempt

This is where most failed attempts go wrong. The vehicle only learns the new code on an ignition transition from OFF to CRANK. If you go straight from ON to START without turning it off first, the relearn doesn't take — and people conclude the procedure doesn't work when they simply skipped a step. Off first. Then start.

If the relearn doesn't work — and sometimes it won't

A relearn only helps when the system needs to re-sync. It can't fix a component that has physically failed. If you've run three full cycles correctly and the light never turns off — or it turns off but the engine still won't start — the fault is likely elsewhere: a worn ignition lock cylinder or Passlock sensor, damaged wiring between the cylinder and the module, or a module that needs programming.

GM's documentation also notes that replacing certain components — the ignition lock cylinder, PCM, BCM, or instrument cluster — requires a relearn afterward. So if a shop just replaced one of those and the car now won't start, that's a known, expected step, not a new disaster.

A Word About What You'll Find Online

Please Don't Do the Resistor Trick

Search this problem and you'll find people bypassing the system with a resistor. Here's why that's a bad trade.

These systems have frustrated GM owners for years, and plenty of forum posts describe wiring around them. We're not going to walk you through it, and it's worth understanding why that's not caution for its own sake:

You are permanently disabling your vehicle's anti-theft system. That's the actual function you'd be removing. It can affect how your insurer views a future theft claim, it's a problem to disclose when selling the vehicle, and improvised wiring at the ignition has its own failure modes — several owners describe bypasses that worked for a year or two and then failed again, leaving them back where they started with extra wiring in the dash.

The underlying fault is usually a worn cylinder, a failed sensor, or a key that no longer reads correctly — all repairable, at your vehicle, without cutting anything out of the car. Fixing it properly costs less than most people assume, and considerably less than a theft claim complication. Call (202) 993-9919 and we'll tell you what it actually is.

Worth Knowing

GM Issued a Bulletin About This

Manufacturer Service Bulletin

"Security Light On Intermittently / No Crank–No Start Or Start Stall And Keyless Access Vehicles May Display No Remote/Fob Detected"

That's the actual title of a GM service bulletin covering an unusually wide swath of the lineup. Two things make it worth knowing about.

First, it confirms you're not imagining it. If your GM has intermittent security lights, random no-starts, or fob detection failures, this is a documented pattern the manufacturer acknowledged — not a mystery unique to your vehicle.

Second, it spans both eras at once — the same bulletin covers old-style security light faults and newer keyless "no fob detected" messages, which is a strong hint that these problems can share underlying causes worth diagnosing rather than guessing at.

Models named in the bulletin range include: Buick Enclave, LaCrosse, Regal, Verano · Cadillac ATS, CTS, DTS, Escalade, SRX, STS · Chevrolet Avalanche, Cruze, Equinox, Silverado, Suburban, Tahoe, Sonic, Traverse · GMC Acadia, Terrain, Sierra, Yukon · Saturn Outlook — across model years roughly spanning 2007 through 2015.

If your vehicle is on that list, it's worth mentioning the bulletin when you call any shop — including us. Knowing a documented pattern exists changes how a problem gets approached.

Newer GM — Different Problem Entirely

"Key Fob Not Detected"
on a Push-Button GM

If your Silverado, Equinox, Sierra, or Terrain suddenly can't find a fob that worked fine yesterday, work through these in order. The first two are free.

Try First — Free

Use the backup transmitter pocket

Every push-button GM has a designated spot where the vehicle can read a fob with a completely dead battery — commonly a specific cup holder or a marked area in the center console. Your owner's manual shows exactly where.

On many models: place the fob flat in the cup holder nearest the dash, then press the brake and start button.

Try Second — Cheap

Replace the fob battery, even if it "seems fine"

This is the single most common cause, and it fools people constantly: a weakening battery often still does remote start from across a parking lot while failing the short-range proximity handshake. Strong remote start does not mean a good battery. Most GM fobs use a CR2032.

Owners on GM forums repeatedly report the problem vanishing after a fresh battery — after assuming it couldn't be the battery.

Then

Try your second fob

If the spare works normally, the problem is the first fob — not the vehicle. That's genuinely useful information and saves diagnosing the wrong thing. If both fobs fail the same way, suspicion shifts to the vehicle's receiver or antenna.

Also Check

Interference and fob location

Phones, laptops, and other fobs stacked together can interfere with the short-range signal. So can a metal-lined bag. Try holding the fob by itself, away from other electronics, before concluding anything has failed.

While We're Here

Remote Start Not Working?
Check the Sequence First.

A surprising share of "my remote start is broken" turns out to be the button sequence, because GM doesn't label it and it's not obvious.

The Correct Sequence

Lock first, then hold the circular arrow

Press and release lock, then press and hold the circular arrow button for at least 3 seconds — some vehicles want 4 or 5. A quick press does nothing, which is why people think the feature failed.

If the exterior lights flash but the engine doesn't turn over, you're pressing correctly and the problem is elsewhere.

Common Blockers

The vehicle is refusing on purpose

Remote start is deliberately blocked by a door or hood not fully latched, an active fault, a low vehicle battery, or having already been remote-started twice in a row without the vehicle being driven. Check the hood latch — it's the one people miss.

Honest Guidance

When It's Worth Calling Someone

We'd rather you fix it free if you can. Here's when it's genuinely past that point.

Three correct relearn cycles, still nothing

If you did all three cycles including turning the ignition off before the final start and it still won't start, the fault is a component — cylinder, sensor, wiring, or module. That needs diagnosis, not another cycle. See ignition repair.

The key looks visibly worn

On these systems a worn key or worn cylinder causes exactly this behavior, intermittently at first and then constantly. Often the cheapest real fix on this page — and worth checking before anything expensive.

Fresh fob battery, still not detected

If a new battery and the backup pocket both fail, and your spare fob behaves the same way, it's a vehicle-side issue rather than a fob issue. See key fob programming.

You've lost keys entirely

All-keys-lost is straightforward on most GM vehicles — new key generated from the VIN, cut, and programmed at the vehicle. Old keys get erased in the process. See lost car keys.

Someone damaged the ignition

Attempted-theft damage to the cylinder is common on older GM trucks and SUVs. Repairable on-site, and the new cylinder can usually be rekeyed to your existing key.

You just want it diagnosed properly

Sometimes the useful answer is simply knowing whether this is a key job, an ignition job, or something for a mechanic. We'll tell you honestly which — including when it isn't ours.

FAQ

GM Security Systems —
Most Asked Questions

Why is my Chevy security light flashing and it won't start?

On older GM vehicles that's the Passlock anti-theft system detecting a mismatch and disabling the start. Often triggered by a worn ignition cylinder, worn key, or a failing sensor. Try the 30-minute relearn first — it's free and resolves many cases.

What exactly is the 30-minute relearn?

Three cycles of: attempt start → release key to ON (not off) → wait ~10 min for the light to stop flashing → turn OFF ~20 seconds. Then turn fully OFF before the final start attempt — the vehicle only learns on an OFF-to-CRANK transition.

I did the relearn and it didn't work. Now what?

First confirm you turned the ignition off before the final start — that single missed step causes most failures. If done correctly and it still won't start, a component has failed: cylinder, sensor, wiring, or module. That needs diagnosis.

Flashing vs. solid security light — different?

Yes. Flashing is the classic Passlock lockout the relearn targets. A solid light with no-start is less likely to clear with a relearn and more often means wiring, sensor, or module trouble.

Should I bypass the system with a resistor?

We'd advise against it. You're permanently disabling your anti-theft system — with implications for insurance claims and resale disclosure — and improvised ignition wiring has its own failure modes. The underlying fault is usually repairable.

My newer Chevy says "Key Fob Not Detected."

Different system entirely — the relearn doesn't apply. Use the backup transmitter pocket (often a specific cup holder) to start, then replace the fob battery. A weak battery often still does remote start while failing proximity detection.

Why does remote start work but the car won't detect the fob?

Because they're different ranges. Remote start is long-range one-way; proximity start needs a short-range two-way handshake that takes more power. A fading battery fails the handshake first — which is why "remote start works, so the battery is fine" is a misleading test.

Can a locksmith fix this, or do I need the dealer?

Most of it is locksmith work — cylinders, keys, fobs, programming, all-keys-lost. Some newest high-security GM models need factory authorization. We confirm by year and VIN before dispatching. Call (202) 993-9919.

Chevrolet Key Service

Silverado, Tahoe, Equinox, Malibu and the rest of the lineup.

GMC Key Service

Sierra, Yukon, Terrain, Acadia and Savana vans.

Buick Key Service

Enclave, Encore, Envision and older sedans.

Cadillac Key Service

Escalade, CT5, XT5 — discreet on-site service.

GM Security & Key Service — DC & Maryland

Tried the Relearn?
Tell Us What Happened.

Call with your year and model, whether the light flashes or stays solid, and how far you got in the procedure. That tells us more than most people expect — and we'll tell you honestly whether it's a key job, an ignition job, or something for a mechanic.

(202) 993-9919 Free Estimate

Ignition Repair

Cylinder & sensor faults

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Lost Car Keys

All-keys-lost, on-site

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Fob Battery

Start with the cheap fix

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Key Fob Programming

All GM fob types