After replacing my spark plugs, the CEL came on, thought it wasn't a big deal and I reset my ECU. Then the CEL came back the next day and I got it scanned and two codes were pulled:

P0031 "HO2S Heater Control Circuit Low - Bank 1 Sensor 1"
P0327 "Knock Sensor 1 Circuit Low"

The car seems to run fine, no startup, idling or bogging issues that I notice. What might be the problem here? It's strange that the codes don't seem related to spark plugs... or could they?
I was thinking that over time, the ECU probably tuned itself into thinking everything is 'okay' between the old worn spark plugs and one of the sensors, but since changing out the spark plugs, the ECU might have realized that there really is a faulty sensor. I'm not sure if my hypothesis makes sense though.

 

Sub'd for a possible answer. A lot of times when codes pop up after you've worked on something they're related. Curious to see what's up here because unless you mixed up coil packs (if you had the car would run like crap if at all if you did) or accidentally disconnected some kind of wire, plug replacement's pretty straightforward and simple.

 

Been doing some research and I've narrowed it down to the upstream passenger side O2 sensor. It seems possible that a bad O2 sensor would cause a knock sensor code due to misfires or something similar. Hopefully that will take care of both codes. Unless the root of the issue is something much smaller to have caused the chain reaction of codes... (perhaps I bought a defective plug?)

Now I just have to decide whether or not I want to try to replace said sensor myself. Just how difficult would this task be? Upstream sensors are in a tight spot and with the age of this car, who knows how difficult it will be to unbolt it...

 

I'd double check your plug installation first and make sure they're gapped correctly. You should get a code if they misfiring though. AllData basically says to just replace the knock sensor if you get that code, check grounds, or the harness. A/F sensor is a PITA to remove; I swapped mine out awhile back trying to figure out a 171 code. You need to unplug and loosen the harness (it's on the top engine bay near the firewall) and then get in there underneath at weird angles to get it out. I ended up buying an O2 set from Harbor Freight and then returned it when I was done. The normal O2 socket I had didn't work well.

 

So as I was getting ready to remove the O2 sensor, it turns out that the problem was a loose connection from the harness to the sensor! Well I sure felt stupid.

Anyway, I replaced the sensor anyway as preventative maintenance seeing as my car is about to hit 140k miles. Reset ECU again and so far no CELs after 30 miles!

Hopefully the codes don't come back and I can say problem solved.

 

lol frustrating yet satisfying outcome. Glad you got it fixed.