[lemansbluen54]

So after a failed downpipe install I put my stock downpipes back in, only to get the codes 2C9C & 2C9D for both precat o2 sensors, both said "oxygen sensor heater before catalytic converter: activation"
I didn't even remove one of the sensors from the stock downpipe. Wondering if it could be the wd40 I used to try and loosen the o2 sensors or a vacuum issue, kind of odd that both went off at the same time though. Car will randomly misfire up top, and will frequently go into limp mode, regardless of my driving style. All help appreciated!

[lookalikehuuh]

Sounds like the pre cat sensor were either damaged or didn't get plugged in properly after installation was complete?

So the 2 connectors for the pre-cat O2s should be on the top of the engine I would start by making sure they are actually plugged in. If they are already plugged in properly ensure the heater circuits are not damaged. This can be done by taking an ohm measurement across the 2 white wires at the connector. I believe you should have somewhere between 10-25ohms a resistance lower or higher that that usually dictates an issue with the sensor itself and will require replacement.

also make sure the connector closest to the front of the engine is black sleeve and the rear one is gray sleeve

[lemansbluen54]

Originally Posted by lookalikehuuh

Sounds like the pre cat sensor were either damaged or didn't get plugged in properly after installation was complete?

So the 2 connectors for the pre-cat O2s should be on the top of the engine I would start by making sure they are actually plugged in. If they are already plugged in properly ensure the heater circuits are not damaged. This can be done by taking an ohm measurement across the 2 white wires at the connector. I believe you should have somewhere between 10-25ohms a resistance lower or higher that that usually dictates an issue with the sensor itself and will require replacement.

also make sure the connector closest to the front of the engine is black sleeve and the rear one is gray sleeve

Would the heater elements failing cause completely random misfires, or could that be something vacuum related

[lookalikehuuh]

Originally Posted by monacobluen54

Would the heater elements failing cause completely random misfires, or could that be something vacuum related

It could cause all kinds of weird issues because 02 sensors only work in a certain temperature range, that is why they have heaters built in to get them up to temp faster than exhaust alone can.

When you start a cold car it runs in "open loop" until the sensors are hot enough to read properly then the car goes into "closed loop" where the O2 sensor is then used to help adjust fuel trims and determine how rich or lean the engine is running.

If the sensors are never getting up to temp properly they could be misreading the mixtures and causing the engine to add too little or too much fuel and then causing the drive-ability issue you have.