Will an Aston Martin DB9 Pass California Emissions Testing After Installing a Secondary CAT Delete Kit?

Secondary Catalytic Converter

If you’ve been following this Blog recently you know I’ve been interested in testing several bolt on performance upgrades for my DB9.  One of those upgrades was a Secondary Cat Delete Kit (SCDK).   You can learn about how I installed it by checking out my other article here.   I wanted to learn if my car would still pass a stringent California Emissions SMOG test with an SCDK fitted.  Read on to find out if it did.


Primary Catalytic Converters

The idea behind a SCDK is to remove the redundant downstream catalytic converter that the car was originally fitted with.  A DB9 has 6 catalytic converters when it leaves the factory.   Four Primaries (two per bank, each serving three cylinders).  Downstream of the Primaries for each bank is a Secondary catalytic converter.

The purpose of these Secondary cats is a bit of a mystery.  There are no oxygen sensors surrounding it, so its not being monitored by the powertrain control module (PCM).   It may be there as a polisher for the exhaust stream.   Or, it may have been fitted by Aston to make the car a Global car able to be sold in other markets that required it.   All this is a guess of course.

Why remove the Secondary Cat?

Breathing is everything in an engine.  Sucking air in, and expelling exhaust out.  It is suggested that removing the secondary catalytic converter will improve three things:

  • Increase engine efficiency and power by removing back pressure the secondary cat creates in the exhaust stream.
  • A slightly more aggressive ‘snarl’ in the exhaust note since the secondary cat acts as a form of muffler.
  • Reduce the vehicles weight by about 10 pounds, further improving handling, mileage and performance.

Is it Bad for the Environment?  Let’s find out.

I live in California, USA – land of the most stringent auto air quality emissions standards in the world.   I wanted to know if the DB9 would still pass a California SMOG test with the SCDK fitted.

To make the test even harder I opted to take it to a Star Certified SMOG test station that looks even that much closer.

Like all Californian’s I’ve had my DB9 SMOG checked every two years since I’ve owned it, and its passed every time.  I keep it in good running order, so I am not surprised.

With my SCDK installed I took my car to a local Star Certified SMOG Test Station.   I didn’t attempt to skew the test at all, I just showed up and turned my car over like anyone normally would.  Sitting in the waiting area and looking through the glass I could see them open the hood, inspect the system, read the labels, stick probes into the tailpipe and even use a mirror to inspect under the car.  Their computer tester did its testing.

They also stopped to pose and take selfies with the car.

Five minutes later I was told it passed with flying colors and they handed me my certificate!  Check it out.

So for $69 USD I have the answer on whether it will pass or not.  Now you know too.

Is it legal?

I suspect the answer is technically a ‘No’ in the United States.   Generally anything that modifies the emission control systems on a stock vehicle would not be looked upon kindly.  This is a blanket position.  If you fitted something that even improved the emission standards it would be considered illegal since it was a modification.  I am just doing all these mods as a test, and will of course return my car’s emission system to stock after the test is complete.

Am I worried about any environmental impact?   Of course.  I don’t want to ‘Roll Coal’ and kill all natures critters.   But I’m not convinced there is any significant additional harm in this change either.  It passed the stringent SMOG test before.  It passed the test after.   I will sleep fine knowing that the sky will still be blue tomorrow and my car won’t be its downfall.  I’m sure you’ll have your own opinion on the matter, and feel free to leave it down in the comment section below.

What’s Next?

With the emission test passed, lets get on with something fun!   Next up with be an article comparing the sound of the exhaust note before and after the SCDK is added.   After that will be a trip back to the Dynamometer to see if its really adds the promised power gains.


Here is a short clip of me being really happy it passed!



10 thoughts on “Will an Aston Martin DB9 Pass California Emissions Testing After Installing a Secondary CAT Delete Kit?

  1. Chris

    Good articles Steve, it will be interesting to see how the test ‘exhaust sound’ went. They abandoned the emissions testing here in Ontario a year ago; I guess they figured out that it was just a money grab and really they weren’t going to save the planet..


  2. Martin Schmidt

    Hi Steve,
    Very interesting project.
    Do you have a list of precise data that florida environmental test requires available so I could compare with the German ones before I start installing secondary cat delete kit?
    Thanks a lot and stay healthy


    1. HI Martin. I don’t have the data about what exactly the California emission test includes. I was just relying on the Test Station to handle all that. Its probably online somewhere, but I’m not sure where. Good luck with trying the kit out in Germany!


  3. Steve Walker

    Your article is inaccurate. They did not stick a probe in you your exhaust pipe. Cars 2000 and newer get an OBD inspection only, no tail pipe monitoring.


  4. Steve Walker

    Could you post a picture of your under hood emissions label? It should state on there how many cats the car has. Wondering if it says 6 or 4. Since you have 2 per side at the engine wondering if they count that as 1 or 2 per side.


  5. David Witham

    Interesting article as always. I see that in California you just get a pass or fail result and they don’t give you the measured values. For international comparison the values would have been handy.
    Removing the primary cats is an option for avoiding any risk of a damaged cat being sucked back into the engine by EGR valve overlap. So it would be good to compare the sound and emissions result with the primary removed and the secondary in place.
    There you go Steve, I have given you and idea for some future articles.


    1. Hi David. Those are good ideas, noted. I might do a primary cat delete in the future. Mike from Bamford Rose and I are doing a more in depth discussion on the Primary Cat failure in a future post. Probably won’t publish for a few months though. Stay tuned


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