CEL / Check engine light after decat (main cat)

Gregmorse · March 30, 2021, 13:18

Hi guys,

So i've tried searching google/this site for an answer many times. Problem is virtually every response is to do with precats. That's not my issue.

I decatted my main Cat around a year ago. As soon as I did this I started getting an error code relating to the fact the sensor was reading an issue with the CAT(cant remember the code was somehting like po4220)

I read somehwere that you can fit a resistor to the heater core that should stop this...it didn't.
I read somehwere else that you can fit a transmitter in the system which transmits a signal to ECU telling it that all is fine, this didn't work either for me.
I read somewhere else that you can fit a spacer to the Oxygen sensor that will fool the sensor into thinking the air is "cleaner". This too didn't work.

So I'm a bit stumped.

My CEL comes on every 2nd/3rd time my car gets "hot"(I mean, I have to drive it for a good 20-30 minutes, @60-70 for most of the time. If i'm only doing short journeys, it doesnt come on every 2nd/3rd journey.

Any ideas on what I should try? I'm willing to give another simulator a go etc but can't even find them anywhere on ebay/the web now.

Thanks in advance.

Carolyn · March 30, 2021, 13:21

I know this sounds daft but.

Why not put a working cat in? Be kind to the environment and all that!

1979scotte · March 30, 2021, 15:17

Why does the CEL bother you?
Just clear it every now and again.

The resistor mod should work if you have the right resistor. People have had issues when the resistor wasn't quite right. Happens with the airbag to.
The defouler spacer probably won't work for a full decat. That's usually for 100 cel cats.
Transmitter to the ECU that sounds a load of bull to me.
And the precats being removed makes no difference to anything 02 sensor related.

Don't really understand the benefits of doing without a cat it's just hassle.
My V6 and Turbo both had cats saved a lot of bother.

Gregmorse · March 30, 2021, 16:08

Quote from: 1979scotte on March 30, 2021, 15:17Why does the CEL bother you?
Just clear it every now and again.

Im not sure if its a Placebo -but i feel the car performing worse right before/whilst the light is on...
Wasn't aware that the defouler wasn't for full decat!
I tried one resistor, then upped both the amps and ohms, or was it Watts..(iirc) which still didn't solve it

I decatted for a little more raspiness and volume from exhaust, I have a TTE dual exit, and with the cat in it sounded nice but without the cat, it's a different noise entirely!

Thanks for your info though...so seems there's no way to get rid of CEL with Cat out totally...hmm...

And its been proven that CEL has no negative affect whatsoever on performance?(i mean, when the CEL is triggered by lack of CAT)

Carolyn · March 30, 2021, 16:13

Quote from: Gregmorse on March 30, 2021, 16:08
Quote from: 1979scotte on March 30, 2021, 15:17Why does the CEL bother you?
Just clear it every now and again.

Im not sure if its a Placebo -but i feel the car performing worse right before/whilst the light is on...
Wasn't aware that the defouler wasn't for full decat!
I tried one resistor, then upped both the amps and ohms, or was it Watts..(iirc) which still didn't solve it

I decatted for a little more raspiness and volume from exhaust, I have a TTE dual exit, and with the cat in it sounded nice but without the cat, it's a different noise entirely!

Thanks for your info though...so seems there's no way to get rid of CEL with Cat out totally...hmm...

And its been proven that CEL has no negative affect whatsoever on performance?(i mean, when the CEL is triggered by lack of CAT)

The main problem with an always-on CEL is - what if you have another issue and the light is already on??

Zxrob · March 30, 2021, 16:48

I have decat mine and get error code too but only after a few good miles, I have wired in a resister and used a defouler and still get the code, I have a bluetooth dongle/reader permanently fixed in the car so just erase when need be, I do check occasionally to make sure no other codes appear

Cant say I've noticed a drop in performance with the light on

Rob

Gregmorse · March 30, 2021, 16:53

The main problem with an always-on CEL is - what if you have another issue and the light is already on??
[/quote]

this is precisely why I wanted to do things "properly" by making sure the engine light doesn't come on because I have no Cat...Hmmm..back to the drawing board!

Gaz mr-s · March 30, 2021, 16:57

The resistor is purely & only if the heater cct fails in the sensor. It throws a specific code. If you read something other than that you're reading in the wrong places....

People have fitted a fouler/extender when dispensing with the main cat & it worked.

There is the occasional claim online that a car without the main cat will pass an MOT, but most knowledgeable folk don't believe it.

Did you intend putting the cat-pipe back in come MOT time.?

paulj · March 30, 2021, 17:02

I had a problem with my post cat sensor a while back, it was also a driving dependent issue and did not flag on short journeys. If I recall, the sensor and Ecu logged the problem but did not flag until it moved from "pending". For my code that meant being present after sustained high revs and at normal engine temperature. If those conditions werent met (local drives) it never materialised as a ECU light. You should be able to check this with a code reader, you will get lists for both confirmed and pending errors.

Joesson · March 30, 2021, 17:04

Quote from: Gregmorse on March 30, 2021, 16:53The main problem with an always-on CEL is - what if you have another issue and the light is already on??

this is precisely why I wanted to do things "properly" by making sure the engine light doesn't come on because I have no Cat...Hmmm..back to the drawing board!
[/quote]

"Properly"!I do believe Mr T already spent some time on "the drawing board".

Ardent · March 30, 2021, 20:02

Quote from: paulj on March 30, 2021, 17:02I had a problem with my post cat sensor a while back, it was also a driving dependent issue and did not flag on short journeys. If I recall, the sensor and Ecu logged the problem but did not flag until it moved from "pending". For my code that meant being present after sustained high revs and at normal engine temperature. If those conditions werent met (local drives) it never materialised as a ECU light. You should be able to check this with a code reader, you will get lists for both confirmed and pending errors.

As described here. The ecu follows a 2 trip logic.
If the "issue" presents in two consecutive trips. You get a light. If it doesn't, you don't.
So as above.
Option 1. If light comes on. Check, reset and carry on.
Option 1. Fit a cat. If light comes on. Check, reset and carry on.

Petrus · March 30, 2021, 20:42

Quote from: Gaz mr-s on March 30, 2021, 16:57People have fitted a fouler/extender when dispensing with the main cat & it worked.

As in my car.

I have to faff a lot for MoT anyway.

m1tch · March 31, 2021, 08:18

Worth noting that with the CEL on, the engine will be running a rich fixed map rather than the usual map, although a full decat won't pass an MOT.