Peugeot 308 Alarm Keeps Going Off: Causes, Fixes & Real Solutions

If your Peugeot 308 alarm keeps going off, you’re not alone. It’s 2 a.m., the street is silent, and suddenly your car screams like it’s auditioning for a horror movie. You rush outside in slippers, key fob in hand, wondering what on earth is going on.

We’ve all been there.

In this in-depth guide, we’ll break down why the alarm keeps triggering, how to diagnose it properly, and what actually fixes it — without wasting money on unnecessary parts. Whether you drive a T7 (2007–2013), T9 (2013–2021), or the latest generation, this guide applies.

Let’s silence that alarm for good.

Understanding the Peugeot 308 Alarm System

Before we start replacing things, we need to understand what we’re dealing with.

The Peugeot 308 uses a factory anti-theft system integrated into its Body Systems Interface (BSI). It monitors:

• Door sensors
• Bonnet switch
• Boot latch
• Interior motion sensors
• Tilt sensor (on some trims)
• Battery voltage

If any of these signals look suspicious, the alarm activates.

The tricky part? A false trigger looks exactly like a real intrusion to the system.

Why Does the Peugeot 308 Alarm Keep Going Off?

Let’s get straight to the heart of the problem.

1. Weak or Dying Car Battery

This is the most common cause.

When battery voltage drops below a safe threshold, the alarm system interprets it as tampering. Voltage instability = alarm activation.

Signs:

• Alarm triggers at night (cold weather)
• Slow engine crank
• Flickering dashboard lights

If your battery is over 4–5 years old, start here.

2. Faulty Bonnet (Hood) Sensor

The bonnet switch is notorious for causing phantom alarms.

If the switch thinks the hood is slightly open, even when it isn’t, the alarm fires.

Quick Test:
Press the bonnet firmly before locking. If the alarm stops triggering, the sensor is likely the issue.

3. Interior Motion Sensor Sensitivity

If you leave:

• Windows slightly open
• A pet inside
• A dangling air freshener
• Loose objects swinging

The motion sensor may detect movement.

Wind gusts can also trigger it.

4. Door Lock Actuator Failure

Door actuators can send false “open” signals to the BSI.

Common symptoms:

• One door doesn’t lock properly
• Central locking sounds weak
• Alarm triggers randomly after locking

This is especially common on older T7 models.

5. Boot Latch Micro Switch Fault

The boot latch contains a tiny micro switch. When it wears out, the car thinks the boot is being opened.

If your boot warning appears briefly while driving, that’s a clue.

6. Water Ingress and Electrical Gremlins

Electrical systems and water mix like oil and fire — not well.

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Water entering through:

• Rear light seals
• Door membranes
• Windscreen edges

Can cause corrosion in wiring or connectors.

Random alarms often follow.

Peugeot 308 Alarm Keeps Going Off at Night – Why?

Nighttime triggers are extremely common.

Why?

• Cold reduces battery performance
• Less electrical activity means voltage dips stand out
• Quiet environment amplifies alarm sensitivity

If it only happens overnight, the battery is the prime suspect.

Step-by-Step Diagnosis Guide

Let’s approach this logically.

Step 1: Check the Battery Voltage

Use a multimeter:

• Engine off: 12.6V is healthy
• Below 12.2V = weak battery
• Engine running: 13.7–14.5V

If voltage is unstable, replace the battery first.

Step 2: Scan for Fault Codes

Using an OBD scanner:

Look for codes related to:

• Door switch
• Bonnet switch
• BSI communication errors
• Voltage faults

Peugeot-specific diagnostic tools provide deeper insights.

Step 3: Disable Interior Monitoring Temporarily

On many models, you can deactivate interior sensors before locking.

If the alarm stops triggering, the motion sensor is at fault.

Step 4: Inspect Bonnet and Boot Switches

Manually press the switch.

Check for:

• Dirt buildup
• Loose wiring
• Corrosion

Sometimes cleaning solves everything.

Common Peugeot 308 Alarm Problems by Generation

T7 (2007–2013)

• Door actuator failures
• Bonnet sensor issues
• Battery sensitivity

T9 (2013–2021)

• BSI voltage sensitivity
• Boot micro switch wear
• Water ingress issues

Latest Generation (2021–Present)

• Software glitches (rare but possible)
• Sensor calibration faults

Can a Key Fob Cause the Alarm to Go Off?

Yes — indirectly.

If the key fob:

• Has low battery
• Sends inconsistent signals
• Has damaged internal contacts

It can confuse the locking sequence.

Replace the coin battery as a precaution.

How Much Does It Cost to Fix?

Here’s a rough breakdown:

• Battery replacement: $120–$250
• Bonnet switch: $40–$90
• Door actuator: $150–$300
• Boot latch: $120–$250
• BSI diagnostics: $80–$150

Always diagnose before replacing parts blindly.

Temporary Fix: How to Stop the Alarm Immediately

If you’re desperate at 3 a.m.:

1. Unlock the car
2. Lock it manually without deadlocking
3. Disconnect battery (last resort)

Be cautious when disconnecting — modern electronics require correct procedures.

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Can You Disable the Peugeot 308 Alarm Permanently?

Technically yes — but not recommended.

Disabling the alarm:

• May void insurance coverage
• Reduces theft protection
• Can create electrical errors

A proper repair is the smarter move.

When the BSI Is the Problem

The BSI (Body Systems Interface) is the brain behind the alarm.

Symptoms of BSI failure:

• Multiple electrical faults
• Wipers acting randomly
• Lights flickering
• Central locking behaving oddly

BSI repairs can cost $400–$800, but it’s rarely the first culprit.

Preventing Future Alarm Issues

We can prevent 80% of alarm problems with simple habits:

• Replace battery every 4–5 years
• Keep door seals clean
• Avoid high-pressure washing near sensors
• Check drainage channels regularly

Think of it like maintaining your immune system — prevention beats emergency surgery.

Is It Safe to Drive If the Alarm Keeps Triggering?

Yes — usually.

But if it’s caused by:

• Electrical instability
• Battery failure
• BSI malfunction

It could eventually affect starting or locking reliability.

Fix it sooner rather than later.

When to Visit a Specialist

Go to a Peugeot specialist if:

• Alarm triggers daily
• Diagnostic scan shows multiple faults
• You suspect BSI corruption
• Water damage is present

Specialists understand PSA electrical systems better than general garages.

Final Thoughts: Silence Restored

When your Peugeot 308 alarm keeps going off, it feels dramatic. Embarrassing. Frustrating.

But the solution is usually simple.

In most cases, it’s:

• A weak battery
• A bonnet switch
• A door actuator

We solve it step by step. Calmly. Logically. No guesswork.

Your 308 isn’t haunted — it’s just sensitive.

Fix the root cause, and peace returns.

Frequently Asked Questions

1. Why does my Peugeot 308 alarm go off randomly at night?

Usually due to a weak battery or voltage fluctuation caused by cold weather.

2. Can a low key fob battery trigger the alarm?

Indirectly, yes. Replace the coin battery to rule it out.

3. How do I disable interior sensors on a Peugeot 308?

Most models allow sensor deactivation via a dashboard button before locking.

4. Is the bonnet switch expensive to replace?

No — it’s relatively inexpensive and often under $100 fitted.

5. Does disconnecting the battery reset the alarm system?

It can temporarily, but the root cause will return unless fixed.

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