cat lights on after doing exhaust system

[7 series 0]

hey guys im new to the forum....i bought a 740i 93 an have done a full exhaust system....have removed cats as well havent put the cat sensors in but have installed the two oxy sensors back in, now the problem is the cat warning has come on...how do i fix this do i have to put the cat sensors back in or is there an easier way maybe bridging wires???

[Greg111 13]

Bridging the wires will make the light go out but that's not necessarily a good fix for this type of thing.

Edited April 14, 2009 by Greg.

[7 series 0]

Bridging the wires will make the light go out but that's not necessarily a good fix for this type of thing.

what kinda problem will it cause????

[*Glenn* 855]

Just bridge the cat overheat sensor wires...it'll be fine on your model car.

[CamB 48]

Since a solution has been provided...

Slightly off-topic:

My wife's Skoda (LOL) has a similar problem (check engine light for non-functioning catalytic converter - largely coz its gone). I haven't got around to trying to get it fixed yet but I understand in her car (which is a 2004 model) there is no easy fix by bridging wires.

Apparently you reposition the sensor using an elbow to move it out of the direct flow of the exhaust and it's sweet as. I am unsure, but it will be cheap enough to try... has anyone heard of (or better) tried such a bodge, err modification.

[Greg111 13]

Just bridge the cat overheat sensor wires...it'll be fine on your model car.

If it's the heat sensor that's gone then it's no worry, if it was the oxy sensor then that can cause all types of wrongness.

[*Glenn* 855]

Since a solution has been provided...

Slightly off-topic:

My wife's Skoda (LOL) has a similar problem (check engine light for non-functioning catalytic converter - largely coz its gone). I haven't got around to trying to get it fixed yet but I understand in her car (which is a 2004 model) there is no easy fix by bridging wires.

Apparently you reposition the sensor using an elbow to move it out of the direct flow of the exhaust and it's sweet as. I am unsure, but it will be cheap enough to try... has anyone heard of (or better) tried such a bodge, err modification.

You should be able to fit a cat overheat sensor into the exhaust, plug it in and the light should go off. Just fit it in the about the same position it was fitted when it had a cat

[CamB 48]

You should be able to fit a cat overheat sensor into the exhaust, plug it in and the light should go off. Just fit it in the about the same position it was fitted when it had a cat

The code is P0420 CATALYST EFFICIENCY BELOW THRESHOLD - is that a different problem/fix?

[*Glenn* 855]

The code is P0420 CATALYST EFFICIENCY BELOW THRESHOLD - is that a different problem/fix?

If it has no cat & no sensor it will automatically store a fault code. If you retro fit a genuine sensor in the exhaust line and plug it in it should be fine. UNLESS it has pre cat & after cat oxy sensors....then a cat MUST be fitted back into the exhaust system or you could end up damaging the engine... it will run too lean.

[CamB 48]

UNLESS it has pre cat & after cat oxy sensors....then a cat MUST be fitted back into the exhaust system or you could end up damaging the engine... it will run too lean.

Its this one, so I should really sort it out. Modification is by previous owner.

[*Glenn* 855]

Its this one, so I should really sort it out. Modification is by previous owner.

I'd get it sorted real quick...going to be expensive though.... but then...so is a new motor.

Some exhaust repairers destroyed a Audi A8 by removing the cats over in Henderson. Took the owner one week of driving to destroy it.

[CamB 48]

I'd get it sorted real quick...going to be expensive though.... but then...so is a new motor.

Some exhaust repairers destroyed a Audi A8 by removing the cats over in Henderson. Took the owner one week of driving to destroy it.

I'm not entirely sure it will cause it to run lean. I would have thought the engine decides how much fuel to use based on the first sensor, and if the second sensor is indicating the cat isn't working (because its not there) that wouldn't have a bearing on the ECU's fueling.

It would be stupid of them to do so - cats can and do lose effectiveness and/or fail and and/or clog, and the engine's fueling requirement shouldn't be dictated by air/fuel ratio downstream of it.

So basically I understand the only job of the second oxygen sensor is to generate a warning that your cat isn't working.

http://www.aa1car.com/library/o2sensor.htm

[*Glenn* 855]

I'm not entirely sure it will cause it to run lean. I would have thought the engine decides how much fuel to use based on the first sensor, and if the second sensor is indicating the cat isn't working (because its not there) that wouldn't have a bearing on the ECU's fueling.

It would be stupid of them to do so - cats can and do lose effectiveness and/or fail and and/or clog, and the engine's fueling requirement shouldn't be dictated by air/fuel ratio downstream of it.

So basically I understand the only job of the second oxygen sensor is to generate a warning that your cat isn't working.

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No thats not right. If it has pre cat & after cat oxygen sensors they evaluate the emmission difference and trim the fuel mixture through the DME. They also determine cat effectiveness, which will indicate a different fault code. How these 2 oxygen sensors work for any particular car is dependant on software, its not a general rule of thumb. Most modern systems now use these 2 sensors to trim fuel. If you remove the cat it will run lean and cause engine damage. Are you sure your car has this system ?... if it only has a pre cat oxygen sensor and a cat overheat sensor your OK. Whats the chassis # of your car ?

Edit: Oxygen sensors will be 4 or 5 wire on your car. Cat overheat sensors are 2 wire

Edited April 16, 2009 by *Glenn*

[CamB 48]

No thats not right. If it has pre cat & after cat oxygen sensors they evaluate the emmission difference and trim the fuel mixture through the DME. They also determine cat effectiveness, which will indicate a different fault code. How these 2 oxygen sensors work for any particular car is dependant on software, its not a general rule of thumb. Most modern systems now use these 2 sensors to trim fuel. If you remove the cat it will run lean and cause engine damage. Are you sure your car has this system ?... if it only has a pre cat oxygen sensor and a cat overheat sensor your OK. Whats the chassis # of your car ?

Edit: Oxygen sensors will be 4 or 5 wire on your car. Cat overheat sensors are 2 wire

It's a 1st generation Skoda Octavia VR-S, OBD-II. As I said, the fault code is P0420 which I understand is the cat effectiveness code.

I am planning on getting it addressed / checked!

[CamB 48]

It's a 1st generation Skoda Octavia VR-S, OBD-II. As I said, the fault code is P0420 which I understand is the cat effectiveness code.

I am planning on getting it addressed / checked!

The plot thins...

I believe I do have a cat, of sorts - just not much of one...

http://www.goapr.com.au/products/18trhd_downpipe.html

Either way, will get the thing seen to. I switched on the Revo software the car came with - have spent a few weeks driving without. Boooooost is fun, even though only set to about 1 bar (can go much higher). Not sure I see the need to go higher. Have also set the timing to a conservative map, as again I don't see the point in going higher...

Edited April 16, 2009 by CamB