First Alert SC9120B vs 9120B: What Is the Difference? (November 2026)

On this page (10 sections)

1. First Alert SC9120B vs 9120B: What does the model number mean?
2. Does the SC9120B detect both smoke and CO?
3. Which SC9120B product fits your replacement?
4. How do the power options compare?
5. Can the SC9120B replace a smoke-only 9120B?
6. What should you check before installation?
7. Why is my SC9120B chirping or flashing?
8. What model replaces the First Alert SC9120B?
9. How do you replace an old SC9120B safely?
10. First Alert SC9120B vs 9120B: Which should you choose?

The SC9120B is the combination model for smoke and carbon monoxide detection, while a 9120B label may refer to an older smoke-only alarm. The model prefix matters because a smoke-only alarm cannot provide the same CO protection as a combination alarm.

The main difference in this First Alert SC9120B vs 9120B comparison is hazard coverage. The SC9120B products below detect smoke and carbon monoxide, while a legacy 9120B without the SC designation may be a smoke-only model.

Before replacing an alarm, read the complete model number on its face or back. Also check whether it is hardwired, battery-powered or connected to other alarms.

First Alert SC9120B vs 9120B: What does the model number mean?

SC9120B identifies a combination smoke and carbon monoxide alarm in the products compared here. A 9120B label without the SC prefix needs closer checking because older model families can differ in detection type and power arrangement.

The “SC” distinction is useful when you are replacing an alarm in a bedroom, hallway or living area. A combination alarm monitors 2 hazards in one device, while a smoke-only alarm focuses on fire smoke.

That does not mean every alarm with similar numbers can be swapped without checking. Connector style, mounting hardware, power source and interconnection rules still affect whether a replacement is suitable.

For a plain-language overview of the device itself, read what is a smoke alarm. It explains how smoke alarms differ from combination smoke and CO alarms.

Does the SC9120B detect both smoke and CO?

Yes, the SC9120B models compared here detect both smoke and carbon monoxide. The supplied First Alert versions use electrochemical CO sensing, and their product descriptions also identify ionization smoke sensing.

Carbon monoxide is a separate hazard from smoke. It can come from fuel-burning equipment and may be present without the visible smoke that would trigger a smoke-only alarm.

The BRK SC9120B also combines smoke and CO detection. Its product information describes a superior CO sensor, dual ionization smoke detection, separate smoke and CO indicators and a single test or silence button.

A smoke-only 9120B does not offer that same combination coverage. If your current alarm has no CO marking, do not assume that a similar model number means it protects against carbon monoxide.

Which SC9120B product fits your replacement?

The best choice depends on how many alarms you need, how the alarm is powered and which backup battery setup you want. These 3 products all carry the SC9120B name, but they are packaged and positioned differently.

First Alert BRK SC9120B-6 Hardwired Smoke

The First Alert BRK SC9120B-6 Hardwired Smoke is a 6-count pack of hardwired smoke and carbon monoxide alarms with battery backup. It includes 6 alarms, 6 mounting brackets and 6 9-volt batteries.

This pack suits a home, rental property or renovation where several hardwired alarms need replacement together. The adapter plugs can help replace existing hardwired alarms without rewiring, but the existing system still needs to be checked for compatibility.

The alarms have a single test button, side-load battery compartments and tamper-resistant locking brackets. Indicator lights identify whether the alarm was triggered by smoke or carbon monoxide.

Choose this pack when quantity is the main advantage. If only one alarm needs replacement, the 6-count package may leave you with more units than you need.

FIRST ALERT SC9120B Corded Electric Combination Alarm

The FIRST ALERT SC9120B Corded Electric Combination Alarm is a single hardwired combination alarm with adapter plugs for replacement. It has a 10-year lithium battery backup, a locked battery drawer and an end-of-life signal.

This is the most direct fit when you need one First Alert SC9120B alarm rather than a multi-unit pack. Its adapter plugs and quick-connect wiring harness are designed for replacing existing hardwired alarms without rewiring.

The alarm has an electrochemical CO sensor and an ionization smoke-sensing chamber. Intelligent smoke sensing is designed to reduce nuisance alarms, and the single button tests or silences the unit.

Choose it when long backup-battery life and a single-unit package matter. Confirm the existing connector and alarm system before installation, especially if other alarms are interconnected.

BRK SC9120B Smoke and Carbon Monoxide Alarm

The BRK SC9120B Smoke and Carbon Monoxide Alarm is a single combination alarm with a 9-volt battery, a universal mount bracket and a battery drawer lock.

Its product information describes 120-volt AC operation, interconnection with compatible BRK and First Alert hardwired alarms and separate visual and audible warnings for smoke and CO. The exact system limits and compatible devices should be confirmed before connecting it to an existing alarm network.

This model suits readers who are replacing a BRK-labeled SC9120B or want the classic BRK version of the combination alarm. Its 9-volt battery setup differs from the 10-year lithium backup in the single First Alert package.

Choose it when the bracket, brand label and existing BRK system are the deciding factors. It is less convenient if you specifically want a long-life lithium backup battery.

How do the power options compare?

The First Alert SC9120B-6 and the single First Alert SC9120B are hardwired alarms with battery backup, while the BRK package is described as battery-powered with a 9-volt battery. Match the replacement to the power arrangement already installed.

Hardwired alarms connect to the home's electrical system and may continue monitoring during an outage when their backup battery is installed. The 6-pack uses 9-volt batteries, while the single First Alert package uses a 10-year lithium battery.

The BRK SC9120B uses one 9-volt alkaline battery. Its product information also describes 120-volt AC interconnection features, so inspect the existing installation rather than choosing by the battery label alone.

A hardwired replacement may need an adapter plug, quick-connect harness or compatible bracket. Do not force a connector into place or alter household wiring without following the installation instructions.

If you are planning the work yourself, start with how to install a First Alert SC9120B. Turn off the correct circuit before removing a hardwired alarm, then test the system after installation.

Can the SC9120B replace a smoke-only 9120B?

It may be a useful upgrade when the wiring and interconnection system are compatible, but the model should be verified before replacement. The SC9120B adds carbon monoxide detection, so it is not simply a cosmetic replacement for a smoke-only alarm.

Compare the old and new alarm labels. Look for the complete model number, power source, connector, mounting bracket and whether other alarms sound together during a test.

The 6-pack and single First Alert versions include adapter plugs intended to simplify replacement of existing hardwired alarms. That convenience does not remove the need to confirm compatibility with the alarms already connected in the home.

For a product comparison involving another possible replacement, see First Alert SC9120B vs SMICO100-AC. It can help if the older alarm has a different label or later manufacturing reference.

You can also check what is compatible with a First Alert SC9120B before connecting the replacement to other alarms.

What should you check before installation?

Check the model label, power source, connector, mounting bracket and interconnection setup before you remove the old alarm. These details determine whether the replacement is a practical match.

Take a photo of the old alarm label and wiring before disconnecting anything. Do not rely on the number 9120B alone because the SC prefix identifies a different detection purpose.

For a hardwired alarm, switch off the relevant circuit and confirm that the alarm is no longer powered. Use the included adapter plug or compatible connector only as directed by the installation instructions.

Install alarms in the locations required for your home and follow the manufacturer's placement guidance. The First Alert product information recommends CO alarms on each level and in each bedroom.

After installation, press the test button. If the alarms are interconnected, confirm that the expected units respond together.

Why is my SC9120B chirping or flashing?

A chirp or flashing light can indicate a battery, power, alarm-memory or end-of-life condition, but the exact signal depends on the model. Check the alarm's instructions before replacing parts or disconnecting the unit.

A low-battery warning can occur on alarms that use 9-volt backup batteries. The single First Alert model also includes an end-of-life signal, which indicates that the alarm needs replacement rather than only a new battery.

The indicator lights can help identify whether a previous alarm came from smoke or carbon monoxide. Do not ignore a CO alarm or treat repeated alarm sounds as a routine battery issue.

If the unit is making an unexplained sound, follow why is my First Alert SC9120B chirping. For a light-related question, see why is my First Alert SC9120B flashing green.

What model replaces the First Alert SC9120B?

The right replacement is a compatible combination smoke and CO alarm that matches the existing hardwired system, connector and mounting setup. A similar model number alone is not enough.

The single First Alert SC9120B is the most direct one-unit option in this comparison. The SC9120B-6 is better when several hardwired alarms need replacement, while the BRK SC9120B suits an existing BRK setup when its power and bracket arrangement fit.

Model availability and production labels can change over time. Read what model replaces the First Alert SC9120B before choosing a newer or differently branded alarm.

If the old alarm is already removed, keep its connector and bracket nearby while comparing the replacement. That makes it easier to identify whether the new alarm is a practical match.

How do you replace an old SC9120B safely?

Replace a hardwired SC9120B with the circuit switched off, the correct connector fitted and the alarm tested after installation. Use the instructions for the exact replacement model.

First, identify the old alarm and confirm whether it provides smoke-only or combination detection. Then check the new alarm's power source, backup battery and interconnection requirements.

Next, turn off the circuit and remove the old unit carefully. Install the compatible bracket and connector, fit the required battery and secure the alarm according to the instructions.

Finally, restore power and press the test button. Test each connected alarm if the system is interconnected.

For the removal sequence, see how to replace a First Alert SC9120B. If the wiring does not match or the circuit is unclear, ask a qualified electrician for help.

First Alert SC9120B vs 9120B: Which should you choose?

Choose the SC9120B when you want smoke and carbon monoxide detection in one alarm. Choose a smoke-only 9120B replacement only when that detection type is intentional and the exact model is confirmed.

Pick the First Alert BRK SC9120B-6 Hardwired Smoke when you are replacing several hardwired alarms and want a 6-count package with 9-volt battery backup.

Pick the FIRST ALERT SC9120B Corded Electric Combination Alarm when you need one First Alert combination alarm with adapter plugs and a 10-year lithium battery backup.

Pick the BRK SC9120B Smoke and Carbon Monoxide Alarm when its BRK label, universal mounting bracket and 9-volt setup fit your existing system.

The key decision is not the similar number. It is whether you need combination protection, whether the power and connector match, and whether the alarm can work safely with the rest of your home's system.

Frequently asked questions

Is the First Alert SC9120B the same as the 9120B?

The SC9120B identifies a combination smoke and carbon monoxide alarm, while a 9120B label without the SC prefix may refer to a different legacy alarm or a smoke-only model. Read the full model number on the alarm and match its wiring, detection type and power setup before replacing it.

Does the SC9120B detect carbon monoxide?

Yes. The SC9120B products compared here combine smoke detection with carbon monoxide detection.

The First Alert versions use electrochemical carbon monoxide sensing, while the supplied product information also describes ionization smoke sensing.

A smoke-only 9120B does not provide the same two-hazard coverage.

Can you replace a First Alert 9120B with an SC9120B?

Possibly, but check the existing alarm's wiring, connector, mounting bracket and interconnection requirements first. An SC9120B adds carbon monoxide detection, yet the replacement still needs to fit the electrical and alarm system setup.

Turn off power before handling a hardwired alarm and follow the installation instructions.

Which SC9120B replacement should you choose?

Choose the 6-pack when several hardwired alarms need replacement, the single First Alert unit when you need one alarm with adapter plugs and a 10-year battery backup, or the BRK unit when its classic bracket and 9-volt setup fit your system. Confirm the exact model before ordering.