You can hack fix P0420 with oxygen sensor spacer; don't try anything else!

I was reluctant to try this hack because I didn't think there was enough space in the gen 3. So first I experimented a bit and tried poisoning the sensor with silicone but that didn't do anything. So then I tried using a low pass filter but it required too many attempts and too much time to find the right values. So I decided to find out if spacers would fit instead of tweaking the circuit further; I got a pair of those spark plug non-foulers (there are gasket and tapered versions; I got the gasket one) from the nearest auto parts store and a drill bit from harbor freight. (I also got a pair of their steel vehicle ramps which turned out to just fit under the front and provide the ideal amount of height to work underneath.) Drilled through one of them, put them all together with the sensor (no gasket between them but I did use a gasket between the exhaust pipe), and turns out there is enough flex in the heat sheild to squeeze it in! Wires aren't happy but they can survive.



This is the drill bit I used; it drilled through the non-fouler surprisingly easily and the size is perfect for the sensor (1/2").



And this worked PERFECTLY! After some city and freeway driving all monitors except EVAP (which can take days to complete) were OK with no DTCs and it passed the smog test!

So if you have a P0420 and are too cheap to replace the cat (and rebuild the engine to fix the oil or coolant burning if you don't want it to kill the new cat) I highly recommend you try this and only this hack because it just works and is pretty easy to do.

If you can wait a week then instead of drilling spark plug non-foulers, you can find oxygen sensor spacers on ebay which are designed for this purpose. You could try an angled one so that the sensor wires won't be squashed against the heat shield. There are also ones that have a miniature catalytic converter built in, but since they work without them it's probably unnecessary.

In my case the smog test needed to be done as soon as possible as it was overdue so it's nice that you can hack fix the CEL and pass the smog test within one day by drilling spark plug non-foulers from any auto parts store.

 

I wonder why they don't do that anymore, as it's pretty easy to hack the OBD system.

 

uh, what? I did not drill the sensor.

This mod failed a year later so I "upgraded" it to an L-shaped spacer from ebay. but it's a dirty hack for turning a blind eye to a messed up combustion/emissions system that needs a complete overhaul. leaking head gasket, burning/leaking oil, bad/clogged cat, blown flange gasket, shit stinks, but too lazy/cheap/ignorant/depressed to bother with it

 

This is the OEM sensor.

Hmm weird, you used to be able to easily find them on ebay with "o2 spacer" or other relevant keywords, but now nothing turns up at all

This is the kit I got, a year ago. The L-shaped one is ideal but it came with another spacer for some reason, but using them together is too much.
Ignore the red silicode mess