A downpipe spacer stops the cel. Why?

I'm looking at a downpipe for my GTI and see that most/all single cat ones will throw a cel if you don't get the software to stop it. I also have read may times that putting a spacer, like the one Forge sells, will stop the cel if you don't get the software.

I would like to know what makes the spacer stop the cel? It looks to remove the O2 from the exhaust flow but how does that stop the error? Does it have to do with the fact that it now has no exhaust gas to measure so you get less signal. Couldn't you just ground the O2 sensor outside and not insert it in the exhaust at all and leave the plug in the pipe? That way it would just sample the outside air giving the same low signal and make the ecu happy?

I am trying to figure this out so after I install a single cat downpipe and change the APR software I already have to the stock program, I won't throw a cel. Just want to make visits to the dealer as trouble free as possible by switching to stock program.

Let me clarify that the spacer we sell is for the diverter valve and is designed to produce the blow-off valve sound.

The spacers used in exhaust systems are totally different, and are designed to remove the oxygen sensor out of the direct flow of the exhaust stream so as to alter the readings of the sensor and to prevent a CEL by allowing the ECU to think that the sensor readings are normal and within spec.

You can buy them at any auto parts store for less than $5, where they are sold as "spark plug anti-foulers". Depending on the size/length you buy, you may need to drill out the inner bore of the spacer to enable fitment of the O2 sensor.





If you don't have the lateral space to use the straight spacers, companies like 42 Draft Designs offer angled spacers.

http://store.42draftdesigns.com/O2-Sensor-Spacer_p_306.html

Mike a have a question for you.

Now since the purpose of the O2 sensor is to determine whether or not the engine is running lean. Wouldn't it be likely that placing it outside of the direct flow of the exhaust system would give a leaner than normal reading and subsequently cause the ECM to richen up the mixture by adding in more fuel causing the engine to run excessively rich?

DarkCloak said:

Mike a have a question for you.

Now since the purpose of the O2 sensor is to determine whether or not the engine is running lean. Wouldn't it be likely that placing it outside of the direct flow of the exhaust system would give a leaner than normal reading and subsequently cause the ECM to richen up the mixture by adding in more fuel causing the engine to run excessively rich?

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No, because the spacer should only be used on a car with a higher flowing catalytic converter or no cat at all, that would otherwise cause the rear O2 sensor to think the car is running richer than normal anyway, so the spacer just brings things back to equilibrium.

You would not use the spacer on a car that has a stock exhaust system or that has O2 sensors operating within normal perams.

Sorry Mike got the spacer companies mixed.

Thanks for helping clarify this some. Guess I need to check for clearance first then check for best option.

Just curious though, if this was put on a totally stock system would you get a cel also? I can't see a reason why you would want to do it just trying to figuring out how stuff works.

So to clarify, use the spacer on a single cat downpipe so you don't get a cel when running stock software. Doesn't effect operation.

ThunderPig said:

Sorry Mike got the spacer companies mixed.

Thanks for helping clarify this some. Guess I need to check for clearance first then check for best option.

Just curious though, if this was put on a totally stock system would you get a cel also? I can't see a reason why you would want to do it just trying to figuring out how stuff works.

So to clarify, use the spacer on a single cat downpipe so you don't get a cel when running stock software. Doesn't effect operation.

Click to expand...

As I mentioned, you wouldn't want to, and there is no need to use a spacer like this on a stock exhaust.

It would only be necessary on an aftermarket downpipe with a high-flow cat or no cat, if the software being used on the car as well didn't already eliminate the CEL.

Stg. 1 software, in most cases, is designed to be used with a stock exhaust system, so using an exhaust with stock or Stg. 1 software will likely throw a CEL, but most Stg. 2 software setups are designed specifically for an exhaust as well, and will likely electronically change the calibration of the rear sensor, if not shut it off entirely, instead of using a mechanical spacer in order to prevent the CEL.