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To Cat or Not to Cat - 1996-2004 Mustang Catalytic Converters

Written By: Chris Bowman

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A high-flow set of catalytic converters frees up the flow of exhaust gasses and allows your Mustang to make more horsepower and torque, without sacrificing reliability.

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When it comes to purchasing a new mid-pipe for your Mustang, one of the first questions asked by most customers is ‘what is the difference between an off-road mid-pipe and one with catalytic converters, besides the price?’ To answer this question, it helps to know what exactly a catalytic converter is and what it does.

American Muscle

A Mustang's Catalytic Converters – The Basics

Catalytic converters have been equipped on all new cars in the United States since the 1975 model year to comply with U.S. Environmental Protection Agency regulations on automobile exhaust emissions. Their purpose is to convert the toxic byproducts of the internal combustion engine--exhaust gases--into less toxic substances by way of a catalyzed chemical reaction for release back into the atmosphere. Catalytic converters or “cats” look like small mufflers and are normally located directly after the exhaust manifolds (a.k.a. headers).

Flowmaster Mustang Catalytic Convertor Set
Aftermarket Catalytic Convertors

Mustang Catted Mid-Pipes – Street Use

As you may have guessed by now, a catted mid-pipe is one that includes catalytic converters. These are ideal for use on street vehicles that require the use of catalytic converters by law; you will need to check your own local and state ordinances for exact regulations.

In general most aftermarket catted mid-pipes feature high flow catalytic converters which provide a significant increase in horsepower and torque over the restrictive stock factory cats, but they keep your Mustang emissions legal.

BBK Catted 1999-2004 Mustang Mid-Pipe
Catted Mid-Pipe

Facts About Catalytic Converters-Street use

  • Convert toxic byproducts from the internal combustion engine into a less toxic substance
  • Required on street vehicles
  • Aftermarket catted mid-pipes increase horsepower and torque compared to restrictive stock factor cats
2000 GT Mustang Cruising Down the Highway

Mustang Off-Road Mid-Pipes – Track/Race Use

Off-road mid-pipes do not include catalytic converters and are intended for off-road use only by the manufacturer. By removing the restrictive factory catalytic converters from the exhaust, your Mustang is able to produce more horsepower and torque, making such a mid-pipe perfect for race applications such as track and drag racing.

They are not for use on street vehicles that require the use of catalytic converters by law; you will need to check your own local and state ordinances for exact regulations. 

BBK 1996-2004 Mustang Off-Road Style Mid-Pipe
Off-Road (Non-Catted) Style Mid-Pipe

Facts About Catalytic Converters-Track/Race Use

  • Off-road mid-pipes do not include catalytic converters
  • Without catalytic converters, your Mustang will produce more horsepower and torque
  • Malfunction Indicator Light Eliminator—MILs—can be purchased to turn off a faulty O2 sensor check engine light
2000 GT Mustang at the Track

Mustang Off-Road Mid-Pipe Accessories

Since an off-road mid-pipe eliminates the catalytic converters from the exhaust equation, the rear O2 sensors won't see enough oxygen particles in the exhaust, so they'll throw a code with an annoying check engine light. There are two ways to turn off the light (this does not stop the code from generating; it only turns off the light). MIL Eliminators are the easiest way to solve this problem for the 1996 to 2004 Mustangs. MIL, which is short for Malfunction Indicator Light Eliminator, plugs in between the rear O2 sensor and the harness to block the codes generated by the rear O2 sensors. The other alternative is to use a handheld tuner, load a custom tune, and then turn off the check engine light.

Mustang Malfunction Indicator Light Eliminator Plugs
Malfunction Indicator Light (MIL) Eliminator Plugs