Showgone
Make your free QR code

See all guides

Cat-back exhaust vs. emissions-related modifications

Cat-Back Exhaust Versus Emissions-Related Modifications

Quick answer: BAR's Smog Check Manual treats exhaust work installed after the last required catalytic converter ("cat-back") differently from work installed before it. Cat-back pipes, such as crossover pipes and oversized pipes, are listed in the manual's Appendix C table without a check mark, so inspectors don't check an EO for them during a Smog Check. Headers, catalytic converters and other after-treatment parts are marked for an EO check. Exhaust noise rules apply separately.

Where the line falls

Here's how Appendix C of BAR's Smog Check Manual sorts exhaust changes:

  • Installed after the last required after-treatment part ("cat-back"), or on vehicles without after-treatment: crossover pipes, oversized pipes and similar. Not marked for an EO check.
  • Installed before the last required after-treatment part: headers, heat risers, oversized pipes and similar. EO checked, gasoline and diesel.
  • After-treatment controls: catalysts, diesel particulate filters, oxygen, NOx, pressure and temperature sensors, and diesel exhaust fluid systems. EO checked.
  • EGR system modifications: EO checked. Aftermarket replacements don't need an EO.

The manual adds that for a Smog Check, inspectors are only required to check EOs as listed in this table. That rule covers what the inspection looks at; it isn't a statement about which parts are allowed.

What this means for your research

Find your last required converter. Inspectors check for the correct number of catalytic converters in their original locations, so know where your last one sits.

Check what's in the kit. Pieces that go ahead of the last converter fall on the other side of the line.

Sound is its own topic. California Vehicle Code Section 27150 requires "an adequate muffler in constant operation and properly maintained" and bars cutouts, bypasses and similar devices. Section 27151(a) bars modifying an exhaust system "in a manner which will amplify or increase the noise emitted by the motor." Section 27151(b) sets 95 dBA or less for exhaust systems on vehicles under 6,000 pounds gross vehicle weight rating (GVWR), other than motorcycles, tested to "the most current SAE International standard."

The Smog Check Referee can't take decibel readings on vehicles with a GVWR above 5,999 pounds. The Referee's page points owners to the weight sticker in the driver's door pillar or jamb. BAR's Smog Check page links to the Referee's citations and noise violations page.

Practical steps

  1. Photograph your exhaust from the engine back, including converter labels.
  2. Keep the receipt, instructions and part numbers for every piece.
  3. For anything ahead of the last converter, read the EO at CARB and confirm your vehicle is listed. If a cat-back piece came with an EO, save that too.
  4. Save copies of what you find.

The Smog Check inspector decides at inspection.

What Showgone can and can't do here

Showgone logs each exhaust piece with its photos, receipt and any EO number, with a link to check the EO at CARB. Showgone isn't CARB, BAR, the DMV or a smog station, and a Showgone record isn't an official certificate.

Add this part to your free Showgone garage.

Not legal advice.

Sources

Date checked: 2026-10-20