So I just recently replaced my cat and O2 sensor cause I had failed smog the previous time, but this time it's a little funky. I'll show a pic, but is there any reason why it would spike up like this on the 25mph? Thanks for any suggestions or info 👍

 

Move out of California.

 

Without data from the vehicle, this will be difficult. But it would seem that your air/fuel ratio is going lean (less fuel, more air) at higher RPMs. This can be caused by an O2 sensor that is misinforming the computer. It can also occur if the intake air measuring device (MAP sensor) is not measuring all of the air that is ultimately making it into the combustion chamber, or it is seeing the incoming air but misreporting it to the computer. Unmeasured intake air often points to a vacuum leak, but vacuum leaks tend to be worse at lower RPMs and better at higher RPMs (because vacuum is highest at low/idle RPMs). Because your test report shows somewhat the opposite of this, it makes me think that your MAP sensor is reporting a certain amount of incoming air to the computer, but it is erroneously low for some reason, so the computer is probably trimming fuel way down based on what it thinks is less air (but in reality is not) - which if all of this is true would lead to a lean condition at higher RPMs. Lean condition (excess air in combustion) + high ignition temps = higher NOx production. A lean condition would both provide the excess oxygen required to produce NOx as well as causing ignition temperatures to be higher.

On my 96, the very first thing I would do would be to reproduce their test using the two RPMs they did, to see what my fuel trims were doing at those two RPM ranges. I have the benefit of simply hooking up a cheap OBD2 device to read this.

If you, or a friend, or a trusted shop does not have the diagnostic equipment suitable for a 94 to see fuel trim data (air/fuel mixture information) you are left in a position of throwing parts at it. For example, inspecting/replacing the MAP sensor and seeing what the next test result is.

You can read voltages directly from the O2 sensor signal to at least determine what it thinks is going on at these RPMs for its voltage signal corresponds to how much free oxygen is detected in the exhaust stream. An O2 sensor under normal circumstances will cycle constantly between .1v and .9v (it can be a bit jumpy when reading with a multimeter, which is sampling the voltage at a fixed interval). But if the mixture is lean the voltage will stay towards the low end (.1v). If the mixture is rich the voltage will stay towards the high end (.9v).

Sorry for the brain dump, but these types of problems require a little diagnostic work.