Aftermarket Exhaust in Montana
Replacing the exhaust system is legal in Montana, but a muffler in good working order is required and cutouts or bypasses are prohibited. There is no statutory 'no louder than stock' rule — the standard is that the exhaust must not produce excessive or unusual noise.
Verified against primary sources · August 2026
What the law says
Replacing the exhaust system is legal in Montana, but a muffler in good working order is required and cutouts or bypasses are prohibited. There is no statutory 'no louder than stock' rule — the standard is that the exhaust must not produce excessive or unusual noise. Removing the catalytic converter is prohibited by federal law (Clean Air Act § 203) regardless of state rules. Montana has no state vehicle inspection.
Specific Limits
- Cutouts
- prohibited
- Muffler required
- Yes
- Louder than stock
- not addressed; excessive noise standard applies
- Catalytic converter
- removal prohibited by federal law
Exceptions & Exemptions
Exhaust system must include a muffler in good working order. Catalytic converter removal or tampering is prohibited by federal law. Exhaust systems cannot create excessive noise that violates local noise ordinances.
Frequently Asked Questions
Are aftermarket exhaust legal in Montana?
What are the exact aftermarket exhaust limits in Montana?
Are there exceptions for aftermarket exhaust in Montana?
Street-legal aftermarket exhaust picks
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