Originally Posted by Tom Jagiella

Background
Most people have been aware for some time that removing or bypassing emissions controls on a vehicle is illegal. In fact, an individual performing such modifications to their own vehicle is also subject to penalty for making such modifications. Specifically, such actions are considered "tampering". However, it is unlikely that many people out there with a JC Whitney catalog and dreams of their own highly customized vehicle have any notion of the scope of the United States Environmental Protection Agency (USEPA)'s definition of tampering.

Just how sweeping is USEPA's definition of tampering? Well, over the years, they have developed an extensive paper trail in response to individual questions. A representative of USEPA was sufficiently helpful to provide a number of memoranda and letters which USEPA has developed to advise the regulated community as to whether the modifications they have in mind are legal.

In a nutshell, there essentially are no legal exhaust modifications. Anything you do to alter backpressure or the time it takes for your catalytic converter to heat up can be considered tampering. USEPA pushes these definitions to the extent that you could plausibly be accused of tampering if you tuned your carburetor to something other than factory specs. Is there good news? Yes. USEPA doesn't intend to make this an enforcement priority. Essentially, that means that they maintain their legal right to fine you, but won't do so unless the political opportunity presents itself.

The following paragraphs detail USEPA's position as it relates to many commonly held ideas among automotive enthusiasts.

The Statute
In the Clean Air Act and the Clean Air Act Amendments of 1990 Congress gave USEPA the authority to regulate mobile sources. In addition to requiring automakers to design automobiles that met increasingly stringent emissions limits, these laws also established provisions to prevent anyone from defeating those modifications. In a letter dated November 14, 1997, Steve Albrink of the Vehicle Programs & Compliance Division of the Office of Mobile Sources of USEPA wrote a letter providing a more accessible interpretation of USEPA's policy than that contained in the official USEPA interpretation, Memorandum 1A (Office of Enforcement and General Counsel, dated June 25, 1974, Mobile Source Enforcement Memorandum No. 1A, SUBJECT: Interim Tampering Enforcement Policy) and subsequent changes to that policy, including fining individuals as well as repair shops. Mr. Albrink's letter states:

The federal tampering prohibition is contained in section 203(a)(3) of the Clean Air Act (Act), 42 U.S.C. 7522(a)(3). Section 203(a)(3)(A) of the Act prohibits any person from removing or rendering inoperative any device or element of design installed on or in any motor vehicle in compliance with regulations under Title II of the Act (i.e., regulations requiring certification that vehicles meet federal emissions standards). The maximum civil penalty for a violation of this section by a manufacturer or dealer is $25,000; for any other person, $2,500. Accordingly, any change from the original certified configuration of a vehicle or engine, or the manufacture or sale of a non-original equipment aftermarket part or system could be considered a violation of section 203(a)(3) of the Act. This would include modifications to the fuel delivery system.
The Policy
So, according to USEPA, pretty much any modification to your vehicle that in any way alters anything from the intake air to the tailpipe exit constitutes tampering. In fact, USEPA has developed an extensive list of specific questions and answers detailing just how extreme this view is.

Here is a sampling of this interpretation:

Can I remove a catalytic converter from a vehicle that is used only for "off-road" driving?

No. The tampering prohibition discussed in Answer #1 applies to this situation as well. The federal tampering prohibition pertains to "motor vehicles," which are defined by section 216(2) of the Act as "any self-propelled vehicle[s] designed for transporting persons or property on a street or highway." A light-duty vehicle manufacturer certifies an engine-chassis configuration as meeting the applicable emissions standards for motor vehicles manufactured in a given model year, and it is not legal for anyone to "de-certify" a motor vehicle for "off-road" use.
[I wonder if NASCAR knows about this?]

Is it tampering to install a dual exhaust system on a vehicle originally equipped with a single exhaust?

Yes. The general rule is that a motor vehicle emission control system (which includes the exhaust configuration) may not be changed from an EPA certified configuration without subjecting the repair shop to liability for violating the federal tampering prohibition. The exhaust system configuration, including the location of the converters, and exhaust pipe diameter and length, are items specified by the manufacturer because engines and some of the associated emissions systems are generally affected by the exhaust system backpressure, which subsequently affects vehicle emissions. The installation of a dual exhaust system with two converters would, therefore, be considered tampering.… It would not be considered tampering to install a dual exhaust system with two converters if the vehicle manufacturer certified an identical engine-chassis configuration for the vehicle model year or newer that includes such an exhaust configuration.

If I pass a standard state inspection test (i.e., a "sniff" or Inspection/Maintenance Test) is my vehicle legal?

No. The testing that the manufacturers perform to demonstrate compliance with Federal standards is more comprehensive and accurate than the idle tests used by the Inspection/Maintenance (I/M) programs. For example, the procedures for light-duty cars and trucks require the vehicles to undergo prescribed sequences of fueling, parking, and operating in order to simulate an average trip in an urban area. Heavy-duty engine testing requires a similar transient mode testing procedure. During these procedures, the emissions are sampled continuously over the entire test cycle for subsequent analyses of specific components by prescribed analytical techniques. These procedures are more representative of in-use operation than any state or locally run idle emissions test.

Idle emissions tests used by state I/M programs are designed merely as screening tests for identifying gross emitting vehicles which have not had proper maintenance (spark plug replacements, air filter changes, timing adjustments, carburetion adjustments, etc.), and not to detect whether specific emission control components are present and operational. The tests usually only involve the insertion of a probe into the tail-pipe of the vehicle while the vehicle is idling and instantaneously measuring the exhaust emissions. These tests are not as stringent or accurate as the Federal test procedures and the results are not comparable. It would be inaccurate to assume that just because a vehicle "passes" an idle test that it could also pass Federal standards.



A Silver Lining?
EPA's version of good news:

However, the Environmental Protection Agency (EPA) has established an enforcement policy, Mobile Source Enforcement Memorandum No. 1A (Memorandum 1A), to provide guidance to the public to reduce the uncertainty regarding potential liability under section 203(a)(3) of the Act for using or selling aftermarket parts or systems, or making adjustments or alterations to parts or system parameters. … Basically, Memorandum 1A states that we will not consider any modification to a certified emissions control configuration to be a violation of the tampering prohibition if there is a reasonable basis for knowing that emissions are not adversely affected. In many cases, durability aging and emissions testing according to the Federal Test Procedure would be necessary to make this determination.
At its surface, this sounds like a reasonable means of complying with the law while retaining the ability to alter a stock vehicle's aspiration and exhaust. However, as the USEPA documents linked above demonstrate, USEPA's idea of a reasonable basis is probably different than most anybody else's. Specifically, USEPA has placed the burden of proof on the individual. The only way an individual is absolutely assured of remaining in compliance after making modifications is to spend about $2,000 or more putting the modified vehicle through the full Federal Test Procedure (FTP) that the automakers use. Anything short of that, while not being an "enforcement priority", can be considered tampering.
USEPA does not even necessarily accept the conclusions of state boards, such as the California Air Resources Board (CARB), which certifies certain aftermarket parts "emissions legal". While it's safe to assume that nobody would fine someone using CARB approved (50 state legal) parts in an approved application, USEPA still reserves the right to consider this tampering.

In short, unless you have the resources to put your finished project (when is a project vehicle ever finished?) through the FTP, you place yourself at the whim of a regulator's judgment. Should that regulator decide that your actions constitute tampering, you will be facing a $2,500 fine or the legal costs of attempting to fight it. If this comes as a surprise to you, you're probably not alone.

Originally Posted by Redneck Z

Thanks! Good find.