Got rid of the CEL last week for P0420 (emissions). Here they stopped doing testing every 2 years but in the past the car would pass the tailpipe test. I would get the P0420 code a few times weekly and just reset it. Also my exhaust smells a bit of rotten eggs so I assume the cat has been going bad, less than 95% efficiency after 120k miles.
I was going to do the sensor spacer but the downstream probe was not coming out of the pipe. Decided to remove the electrical connector from the firewall (2nd from the left) pulled back the yellow sleeve, cut the signal + lead (black wire) and soldered in a rectifier diode (diode line/neg points to firewall/ECM). The diode lowers the signal voltage by 0.45 volts, eliminating the big 0.1-0.95 volt swings. No more codes after 2 weeks of daily driving.

Also finally got rid of the P0015 camshaft retarded code after cleaning/unseizing the VVT oil solenoid valve plus gasket screen and resetting my camshaft tension-er correctly.I thought I had it with the valve clean-up but as it turned out the tension-er was not set correctly, not centered between the tabs. Also been 2 weeks, no CEL.

Also had a P0171 lean condition but that turned out to be bad hoses coming from the fuel pressure regulator to the intake. The S shaped hose assembly had both rubber pieces badly cracked on either side of the hard plastic tube. I had a spare from when I bought a new FPR a few years ago.

This should help some who are still struggling with these issues.

 

Thanks for that diode trick - I've used the spacers before, but I like that approach as a back-up / alternative.