Catalytic converter and smog check in California, what passes and what fails

Catalytic converters and smog checks in California

The catalytic converter is the part that cleans most of the harmful gas out of your exhaust. California smog inspectors look for it on every test. They do not just check that it is there. They check that it is the right one.

The rule is simple to say and hard on buyers. Since January 1, 2009, CARB has required every aftermarket catalytic converter installed in California to carry an EO number. If the number is missing, or the converter is wrong for your car, the car fails the catalyst visual inspection. This is the number one trap for people who buy a cheap converter online.

This guide covers CARB approved converters, why non CARB ones fail, what to do after a theft, the P0420 code, and the costs we can verify. See what happens during a smog check for the full test.

What an EO number is

EO stands for Executive Order. CARB, the California Air Resources Board, reviews each converter design. When it approves one, it issues an order and gives it a number. That order lists the cars the converter may be used on. The number is stamped or printed on the converter shell.

CARB says the EO number lets anyone look up the approval status of a converter on its website. The label has to be large and clear. CARB's rule is half an inch high, readable from five feet away on either side of the car. You can look up an EO in CARB's aftermarket catalytic converter database.

CARB also keeps a summary list of every company granted an exemption to market new aftermarket converters under the 2009 rules. It was last updated July 21, 2026. It states that only converters that meet the rules can be sold and installed on the right vehicles in California. If a website will not ship a converter to California, this rule is why.

Old labels

Converters sold before January 1, 2009 carry an older stamp that starts with a letter, then CA, then a two letter maker code. You may still see those on older cars. The Smog Check Manual says inspectors use the general procedure for an older converter with no EO label.

Why a non CARB converter fails the visual check

The smog inspection starts with a look at the emissions parts. The manual tells the inspector to check that every needed catalytic converter is present, installed in the original spot, and free of tampering or defects. The number of converters must match too. A change in the count is only allowed when a CARB exemption order says so.

Then the inspector reads the label. For a 1996 and newer car, the inspector uses the EO number and part number to confirm the converter is approved for that exact year, make, model, engine, weight rating and fuel type. The inspector checks CARB's database, and each EO has a catalog of the cars it covers. For an older pre OBD II car, the inspector checks that the EO falls in the right vehicle category.

Notice what is not on the list. Nothing here depends on how clean the car runs. A universal converter without the right EO can fail the visual even if the tailpipe is spotless. BAR and the Smog Check Referee also name aftermarket or wrong converters as a common cause of failure. One case they point out is the 2000 to 2007 Honda Accord SULEV, which needs the original equipment converter.

In 2025, 134,222 cars with an OBD style test failed the visual part, or 1.28% of those tests. That number covers every emissions part the inspector looks at. BAR does not split it out for converters alone.

Can you pass smog without a catalytic converter

No. A missing converter is a visual failure and counts as tampering. A converter that was gutted, drilled or damaged so it no longer works also fails. When an inspector finds that a converter is damaged or modified enough to hurt its function, the inspector enters Tamper or Modified into the system and tells you the car failed. You may need a new converter.

The same goes for a cut and capped pipe, a test pipe or a straight pipe. There is no workaround. A pass needs a correct converter in the correct place.

Theft and replacement

Catalytic converter theft has hit many owners. BAR wrote in its 2021 newsletter that thefts had risen at an alarming rate and left owners with costly, slow repairs. It also warned that homemade theft devices can lead to a smog fail.

Rules for a replacement

• Use OEM or CARB approved. A replacement must be an original equipment part or a CARB approved aftermarket part.
• Show the EO number. The aftermarket converter must be installed so the EO number can be seen on the shell.
• Match your car. The converter must be validated as correct for the vehicle in CARB's database.
• Keep the original layout. It goes in the same location and configuration, with the right number of converters. This matters most for converters that are welded in.

Theft guards

Any licensed automotive repair dealer may fit a guard such as a steel cage, chain, cable or plate. BAR says the guard must not be welded to the converter, must leave the ID numbers visible and must not block needed airflow. If a guard hides the EO number, the inspector has to talk with you about options, including taking the guard off for the test. A guard that damages the converter can cause a fail.

If you disagree with a result and want a third party view, you can ask the Smog Check Referee. See the Referee guide. The Referee handles converter label cases directly.

Check your warranty first

A converter that fails on a newer car may still be under warranty. Federal rules cover the catalytic converter and engine computer for 8 years or 80,000 miles. Other California coverage can apply. Ask the dealer before you pay.

P0420 and other converter codes

P0420 is the standard OBD code for a catalyst that is not cleaning as well as it should. A check engine light with that code usually points at the converter or at a sensor that reads it. A car cannot pass with the light on. BAR is plain about this, and the car also fails if the light stays on or flashes with the engine running.

Do not just buy a converter because you see P0420. A bad oxygen sensor, an exhaust leak or an engine that burns oil or runs rich can trigger the same code and can ruin a new converter. Have a shop test the converter and the parts around it first. For the light, read check engine light and smog check.

The catalyst monitor must be ready

After a new converter, the car needs to finish its self tests. For 2000 and newer gasoline cars, BAR allows only the evaporative monitor to be incomplete. That means the catalyst monitor has to be ready. BAR also notes that the oxygen sensor monitor after the converter may not run until about 1,000 miles of break in on some models. So drive normally for a week or two after the repair, and then test. Our readiness monitors guide explains the details.

What it costs and what we can verify

We do not quote a price for a converter. BAR and CARB publish no price list, and cost swings with your car, whether the converter bolts on or is welded, and the shop. Ask for a written estimate. California requires one before the work starts.

What BAR does publish is the overall repair picture. In 2025 stations logged 46,033 emissions repairs with an average cost of $756.68. About 39% ran over $650. These figures cover every kind of emissions repair, so a converter job sits inside them.

If the bill is too high, look at CAP repair help, which pays up to $1,450 toward repairs for income eligible owners, or at vehicle retirement.

Questions to ask before you buy a converter

• Does it carry an EO number for my year, make, model and engine?
• Is it a direct fit part or a universal one? Either may be legal if the EO lists your car.
• Can you give me the EO and part number in writing?
• What warranty does the part carry?
• Will the shop fit it in the original location?