How to cure CEL from removing primary cat

[Mazdawarbird]

How to cure CEL/MIL from removing CAT

NOTE: This trick also works if your primary CAT fails and throws a CEL (helps pass emissions tests )

This will cure the CEL caused by removing the Catalytic converter, usually when you install headers.

Total time required: 15-20 min.

Tools and Parts required

Tools:
Adjustable wrench & vise grips or another adjustable wrench (you need 2)
Drill with 1/2 inch drill bit
Clamp if available, (Vice Grips and a foot will work too, if you have no clamp)

Parts required
2 Spark Plug Non-fouler adapters 18mm w/gasket (Napa Part 7302442)
2 come in a pack.

Here is a Pic of them in the package.



Step #1
Disconnect the negative terminal on the battery, then Take ONE & only one of your 2 non-foulers and Drill out the Top end with the 1/2 inch drill bit until it goes all the way through.



You can either clamp the piece down to be drilled by hand, or do what I did & clamp some vice grips onto it, hold it down with your foot & drill away. You can also use a Drill press if you have access to one, but its easy to do it with a hand held drill.





Once done it should compare to the un-drilled piece like so...



Step #2
Attach it to the un-drilled piece making sure the Un-drilled piece screws On Top of the drilled piece & making sure there is a gasket on.

It should look like so...





Step #3
Unclip the o2 sensor clip & Disconnect the o2 sensor (2nd o2 sensor for removal of Cat 1 with headers install) from the car using the adjustable wrench.

IMPORTANT to unclip the o2 sensor first so that you can spin the wires while you unscrew it to insure you don't over twist the wires.

Step #4
Screw the o2 sensor into the non-foulers making sure you have a gasket on using either 2 adjustable wrenches or 1 adj wrench and vice grips. It should look like so...





Step #5
Reconnect the o2 sensor to the car and plug it back in. It should look like so...



Step #6
Reconnect the negative battery cable, while we did the install the ECU was reset from us disconnecting the negative battery terminal in Step #1.

YOU'RE DONE!!!!

The ECU should take around 150mi to adjust.

[smokstac]

I can vouch that this works as my 97 was throwing a code saying that the pre-cat was out of spec. Installed the spacers on the downstream O2 sensor and no more code being thrown.

[89BG 323]

i wonder how this fools the ecu into thinking that theres a cat there?

[progt91]

it gets far less flow so it gets a cleaner reading and therefore believes a cat is still present

[89BG 323]

dope! you wouldnt be able to pass emissions testing with this, tho, would you?

[smokstac]

The way the ECU uses the O2 sensors is as follows:

Cold engine -> ECU bypasses the primary O2 sensor and enriches the mixture until the coolant temperature reaches a certain temp

Warm engine -> ECU read the voltage from the primary O2 sensor to determine if more or less fuel is needed to keep the air/fuel ration as close to perfect as possible (all other things considered)

Pre-1996 vehicles have only 1 O2 sensor per bank of cylinders. Some even just had one for all the cylinders...the O2 sensor being mounted on the crossover pipe prior to the cat.

Post 1996 vehicles (OBDII) require cat monitoring and this is done by having a second O2 sensor after each cat...a V6, for example, would have 4 sensors while a 4 cylinder has only 2.

A proper working O2 sensor produces a graph that looks just like a roller coaster....up, down, up, down, etc. as the ECU quickly alters the fuel injectors to maintain a perfect air/gasoline ratio.

The ECU looks at the voltage from the second sensor, compares it with the first, and uses those values to determine if the cat is working or failed.

If both sensors are working but there is no cat (or the cat has failed completely) then the graph of each sensor will produce a waveform that looks almost identical...because nothing is happening to the exhaust from sensor 1 to sensor 2.

A perfectly working emissions system will produce a graph from the first O2 sensor with big troughs (like the ocean in a hurricane) and a graph from the second O2 sensor that approximates a flat line (very small troughs)...like a nearly still pond.

If your cat is dead or removed (because you put in a header and had to remove the cat) then your ECU will throw a code. By installing the spark plug extenders and then putting the O2 sensor on the end it causes enough variance in the exhaust gas sampling to fool the ECU. Even if you hook up OBDII test equipment it will show a normal graph.

In states where emissions testing doesn't include a physical exam then you'll pass every time because with OBDII if the ECU doesn't throw a code and the warm up tests in the ECU have passed then you're good to go (clearing the ECU isn't good enough because the emission's testing station will know you cleared the ECU due to the internal ECU tests not having completed).

That's why headers aren't technically street legal because by removing your cat you are breaking the law and in no state is it legal to remove the cat. But the extenders will help you pass inspection in places where physically checking to see if the emissions equipment is there or not isn't part of what you need to do to pass. In OR, for example, where I used to live the techs at DEQ never opened the hood of the car or looked underneath. If the CEL was off, and, the emissions tests had run already on the ECU then that's all you needed for passing in the uppe Willamette Valley (PDX). On older cars it was harder because on pre-OBDII vehicles a physical exhaust pipe test was run.

[goldstar]

smokstac, good explanation and thanks for posting. Very informative.


In the tradional zirconia type upstream O2 sensor the voltage transitions from ~ .2V when the A/F mixture is lean to ~ .9V when it's rich. Normally, when the catalytic converter is functioning properly the downstream O2 sensor should show little activity (very few lean-to-rich transitions) with a fairly steady voltage with a mean of ~ .45V. When the upstream and downstream O2 sensor outputs start to resemble each other it means converter efficiency has dropped off.

The point at which codes are set is determined by the vehicle manufacturer as part of the emissions compliance testing required for new vehicles. Some set their trigger points conservatively to minimize the risk of an emissions recall while others allow a greater drop in catalyst efficiency before the MIL is trigggered. Consequently, depending on the vehicle, the converter may not be operating at peak efficiency but is still functioning well enough to pass a tailpipe emissions test. Federal law requires the MIL to illuminate if the tailpipe emissions exceed the limit by 150%.

Happy Motoring!

[smokstac]

Here's a graph I made when I had my Passport. The left shows the upstream sensors, the right the downstream sensors. This was a V6 with two cats (one on each bank and had an O2 sensor before and after each cat).

When you have no cat or a bad cat then the right side will almost mirror the left 100%. It's this 100% match that throws the code.

As you can see the right bank cat (lower graphs) is more efficient than the left bank (upper graphs) since the right cat's secondary O2 sensor approaches more of a straight line. In any regard you see the peaks and valleys of the upstream sensors...this is what you want.



If you graph a Protege it would show one upstream and one downstream sensor.