Join Date: Aug 2009
Location: Baton Rouge, LA
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I want headers but what about the CEL?
Well after explaining the CEL and header predicament in our cars a few times I figured it would be a good idea just to create a thread that can be referred to.
First off, lets learn about what 2 key components are critical when it comes to headers and a CEL, catalytic converters and oxygen sensors.
What is a catalytic coverter and how does it work?
A catalytic converter is a device that uses a catalyst to convert three harmful compounds in car exhaust into harmless compounds.
The three harmful compounds are:
Hydrocarbons (in the form of unburned gasoline)
Carbon monoxide (formed by the combustion of gasoline)
Nitrogen oxides (created when the heat in the engine forces nitrogen in the air to combine with oxygen)
In a catalytic converter, the catalyst (in the form of platinum and palladium) is coated onto a ceramic honeycomb or ceramic beads that are housed in a muffler-like package attached to the exhaust pipe. The catalyst helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen.
What is an oxygen sensor and how does it work?
Every new car, and most cars produced after 1980, have an oxygen sensor. The sensor is part of the emissions control system and feeds data to the* engine management computer. The goal of the sensor is to help the engine run as efficiently as possible and also to produce as few emissions as possible.
*A gasoline engine burns gasoline in the presence of oxygen . It turns out that there is a particular ratio of air and gasoline that is "perfect," and that ratio is 14.7:1 (different fuels have different perfect ratios -- the ratio depends on the amount of hydrogen and carbon found in a given amount of fuel). If there is less air than this perfect ratio, then there will be fuel left over after combustion. This is called a rich mixture. Rich mixtures are bad because the unburned fuel creates pollution. If there is more air than this perfect ratio, then there is excess oxygen. This is called a lean mixture. A lean mixture tends to produce more nitrogen-oxide pollutants, and, in some cases, it can cause poor performance and even engine damage.
*Th*e oxygen sensor is positioned in the exhaust pipe and can detect rich and lean mixtures. The mechanism in most sensors involves a chemical reaction that generates a voltage. The engine's computer looks at the voltage to determine if the mixture is rich or lean, and adjusts the amount of fuel entering the engine accordingly.
The reason why the engine needs the oxygen sensor is because the amount of oxygen that the engine can pull in depends on all sorts of things, such as the altitude, the temperature of the air, the temperature of the engine, the barometric pressure, the load on the engine, etc.
When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio, so it ends up guessing. Your car performs poorly and uses more fuel than it needs to.
-Sourced from howstuffworks.com
What is a pre cat or manifold cat?
There are three catalytic converters in our cars. Two of them are located in the exhaust manifolds and the third is located uderneath the car as part of the "Y" pipe. The precats are the ones located in the exhaust manifolds they get hit with the exhaust first and then the third cat under the car cleans up whatever they do not filter out.
Where are the oxygen sensors?
You will find in our cars that there are four oxygen sensors. There are two upstream and two downstream. The upstream ones are located at the top of the exhaust manifolds before the "pre cats" that are built into the stock manifolds, they are known as the "primary O2 sensors". The signals from these are what the ECU uses to determine what adjustments need to be made for the ideal air fuel ratio. These sensors should always be replaced with OEM ones and should never be modified as they play a key role in keeping your engine running healthy.
The "downstream" oxygen sensors or "secondary" oxygen sensors are located just after the pre cats. The signals they send to the ECU are used in conjunction with the primary O2 sensors to determine if the pre cats are operating efficiently. They do this by reading the differences between the two sets of sensors. These O2 sensors are not critical to engine performance they are just there for emissions purposes although they should still always be replaced with OEM sensors.
So why do I get a CEL with headers?
The reason the CEL appears with headers is that when you install headers you remove the pre cats when you remove the stock exhaust manifolds. When you do this there is no longer anything filtering in the exhaust stream between your primary and secondary O2 sensors. The ECU then sees the same readings at both the primary and secondary sensors, it then triggers an emissions related CEL because of the lack of filtration. These codes are P0420 (Catalyst System Efficiency Below Threshold (Bank 1) and or P0430 (Catalyst System Efficiency Below Threshold (Bank 2).
Will these codes hurt my car or it's performance?
No, these codes are emissions related and have no bearing on fuel or air delivery to your engine. The only issues are that you cannot pass an OBD2 inspection with them on and if for some reason something else goes wrong with an unrelated part of the engine that may harm your engine, you won't know because the CEL will already be on. If you have no OBD2 inspection and you can pass a visual inspection the only thing you may want to do is periodcally pull the codes from your car to make sure nothing else is wrong in any other areas.
How can I get rid of the CEL?
With some of the older tibs (06 and older) you may not get a CEL with headers because the emissions sytems (ECU and O2 sensors) are not as sensative. If you do, installing a high flow cat under the car and relocating the secondary O2 sensors behind it may get rid of this issue.
If you have an 07-08 the problem is much more of a pain to deal with. There is no chance of not getting a CEL with the 07-08 models. If you add headers on these models you will get a CEL. Getting rid of it is a pain as well. A normal high flow cat will not do the job, the emissions sytems are far too sensitive. You will need to buy a high end cat (random Technologies for instance) and relocate the O2 senosrs behind that and even then its not a guarantee.
The reason for no guarantee is that although those higher end cats may get rid of the ineffiency codes they may give you a code for O2 sensor heated circuit or O2 slow response. The reason for these is because the O2 sensors also signal the ECU as to how fast the cat heats up. If the new cat heats up way out of range that the stock cats would have, the ECU will aslo throw a CEL. The slow response is just as it sounds, a response back to the ECU from the O2 sensor that is slower than what stock would be.
The best bet with an 07-08 is not have headers in a state that does an OBD2 inspection or just be prepared to take them off every year.
Can't I just reset my ECU before inspection?
No, if your state does an OBD2 inspection they will also be checking for incomplete ECU cycles. If there are too many incomplete your car will come back as "not ready for inspection".
What is an ECU cycle?
An ECU goes through many "cycles" after the ECU is reset to "learn" all the perimeters it should set itself at to operate as efficiently and cleanly as possible (maintaining the ideal A/F ratio). Each emissions system (EVAP, O2 sensors etc.) has to see certain driving conditions to be able to set itself, this can take a few hundred miles to complete. For instance in New York State you can only have 1 ECU cycle incomplete in order to be ready for inspection.
Can I use an O2 spacer?
They may work on an 03 or 04 and its hit or miss. On anything newer than that they will not work.
Can I hang my secondary O2's out in fresh air?
Again, maybe on an 03 or 04, anything newer than that and you will get an "O2 sensor inactivity" code.
Are headers legal?
In a short answer no. Federal guidlines mandate that no changes should be made to any emissions systems on any vehicle until that particular part fails. When that part fails, it should be replaced with an OEM equivalent part.
Which bank is which?
The 3 cylinders on the firewall side of the car are known collectively as bank 1, the 3 cylinders toward the front of the car is bank 2.
This write up is to give a quick explaination and understanding of the CEL related issues with our cars and headers. Be sure to check with your local and state laws and regulations regarding inspections to see what your options are.
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Last edited by Will51; 03-14-2011 at 01:07 PM.