P0420 diagnosis

[sontakke]

Some places still do tail pipe with the smog check.

If your vehicle is running fine otherwise, it will pass the tailpipe emission test with fine colors even with active P0420 code. The ECU is way too stringent than the federally mandated FTP. I used to have reference paper done by U of Colorado where they actually tail pipe tested cars with P0420 code and all were well within the limit,

Once the spacer is there, it essentially becomes indistinguishable from the sensor. Heck, they become inseparable even when you want them i.e. when you need to replace the rear O2 sensor (because its heater dies), you will end up having to purchase new spacer.

Yes, if somebody is looking specifically for it, they will be able to spot it but to the causal observer who is only looking for presence of the converter, he will not be able to notice it. Besides, most state inspection only hook up the code reader and call it a day.

 

[Mark R]

I recently sold this car, so I thought I'd post a final update.

After the cat and everything was changed, things were fine for about a month and then the code came back. Through a friend-of-a-friend I met a guy who was a guru level specialist for a major ECU/emissions OEM, and asked him for advice.

His answer was "check the B1S2 O2 sensor again". So, I did, this time I did it myself and stripped the car down - seats out, carpet out, etc. What I found was slight damage to the wiring harness at the connector where the O2 sensor plugged into the car's harness. I repaired the damaged wire/connector, and the code went away again. The other thing that I noticed was that if you look at the "repaired" trace I posted above, you can see some high frequency noise in the post-fix trace during the period of steady cruise. This noise disappeared after the repair.

So, I presume the noise was coming from a slightly loose connection, and it was enough to cause the ECU to think it was rapid switching of the O2 sensor, but not enough to trigger a short-circuit/open-circuit indication. Quite why it got better for a bit after teh cat change, I don't know. Presumably, the sensor had been disconnected and reconnected, and that tweak had improved the connection.

Summary: It was the wiring harness for the B1S2 O2 sensor.

 

[sontakke]

Wow; that is quite interesting!

 

[GDI Tech]

If your vehicle is running fine otherwise, it will pass the tailpipe emission test with fine colors even with active P0420 code. The ECU is way too stringent than the federally mandated FTP. I used to have reference paper done by U of Colorado where they actually tail pipe tested cars with P0420 code and all were well within the limit,

Once the spacer is there, it essentially becomes indistinguishable from the sensor. Heck, they become inseparable even when you want them i.e. when you need to replace the rear O2 sensor (because its heater dies), you will end up having to purchase new spacer.

Yes, if somebody is looking specifically for it, they will be able to spot it but to the causal observer who is only looking for presence of the converter, he will not be able to notice it. Besides, most state inspection only hook up the code reader and call it a day.

Correct. Only OBDII readiness would fail if they plug in.

Mark, bank 1 sensor 2 is only the cat readiness/effectiveness test. Will fail emissions, but running should not be affected. Sensor 1 on each bank is what the ECU/PCM will use along with IAT, MAF, MAP data to commend fuel, timing, etc.

Glad I live in a "non testing" State.

 

[sontakke]

THANK YOU!! I get tired when people (or even mechanic) start replacing the post catalytic O2 sensor to fix P0420. A big forehead slap :-(

Getting validation from you means a *LOT* to my ego. For more than last 10 years and with at least four different cars, I have been battling this for myself.