No O2 sensor after cat converter

[David]

I have a 98 Honda Civic which I recently straight piped with aftermarket headers I do not have a catalytic converter I do have the O2 sensor that would go before a cat plugged in but I do not have a sensor plugged in that would go after the cat. My question is am I going to cause harm to the engine by doing this or will I have any problems later on because of this? Thank you for your help

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[Mike]

You will not harm anything, although It will generate a catalyst efficiency code if nothing else is done. The rear o2 exists only to monitor the catalytic converter efficiency and has nothing to do with how the engine runs or making fuel trim corrections. The code only “matters” if you need to have to pass a OBD2 inspection to keep your car on the road in your state. Just noticing you are in Cali, it’s my impression that tailpipe emissions are tested there. Not having an actual catalyst installed may prevent it from passing a actual measured emissions test on a dynamometer.

I’ve heard of using resistors/devices that you connect instead of connecting the o2 sensor. I’ve heard them called o2 simulators.

What I see in real life far more often is use of a device that holds the existing o2 sensor out of the direct exhaust stream and shrouds it in a tube, making it extremely slow to respond compared to the upstream. The computer interprets this as proper catalyst functioning and no code is generated. Sometimes the aftermarket pipe has the extension tube built into it. If not, the common thing to do is to buy a pair of 18mm spark plug anti-foulers (a mega cob-job device in it’s intended use). You drill the bottom on one out and leave the other one alone. Thread the drilled one onto the o2 sensor, then thread the stock one onto the drilled on, then install the assembly into the o2 bung on the exhaust pipe. It’s a lot longer now, so if the bung is facing an obstruction it can be a problem. Most of the aftermarket pipes that feature this “mod” built in have them on a 90° elbow to alleviate that fitment issue. This trick I’ve explained usually works, but it occasionally does not work for whatever reason. I’ve never owned a car that’s had this done to it, so my experience is limited to all the modded Mitsubishis and Hondas I deal with.

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