Interesting. Let us know after a while if it continues to no longer have the P0420.

 

I suspect it will come back on in a few drive cycles.

 

Nailed it - CEL back on the next day after a short drive. I was hoping it was just fouled from burning oil. Any suggestions on the next step? Bad sensor or bad catalyst or something else?

 

I was hoping it was just fouled from burning oil.

I think that is what the issue is. Cat and/or rear O2 sensor are fouled or coated with soot. Compressed air is not going to resolve that. Needs to be chemically cleaned (if the cat and sensor aren't ruined).

 

Code po420 means bad catalysts which has to do with the cat converter. The cat converter is out of its threshold.
To see if the cat converter is bad: you have to graph both the before and the after oxygen sensor. A good oxygen sensor in the presence of oxygen would be switching back and forth when graphed. A bad oxygen sensor will be flat lined when graphed.
The oxygen sensor after the cat tells you if the cat is good or not. So when both oxygen sensors are graphed if the after cat sensor is switching back and forth like the front the cat is bad.
It the after cat oxygen sensor when graphed is flat lined the cat is good.

 

Some causes of a P0420:


Exhaust leak
Air intake leaks
Oil/fuel burning (coating on catalyst media) - This could be from the overfilling and wrong fluid in the crankcase.
Faulty downstream O2 sensor
Plugged or partially plugged up catalyst media
"Worn out" or no longer functioning catalytic converter
Fuel injector problems


Let's start with the elephant in the room: You had your crankcase overfilled (by a lot) with oil AND with a lot of transmission fluid. This caused burning/smoke out of your exhaust manifold and has coated your catalytic converter and/or your O2 sensor. You can start by taking a gallon of pure lacquer thinner ($14 at WalMart) with you to the gas station when you're near empty on gas. Put the whole gallon in your tank and then fill with gas to about 1/2 tank full. Get on the freeway and drive at freeway speeds for a couple or few hours (go visit your family somewhere). If you had a bunch oily smoke coating your cat, this should clean it up.


Secondly, use a scan tool to see how the rear sensor is reacting to things. I just went through this on my Corolla with 372K miles, which finally has a bad cat. If the rear O2 sensor is moving around a lot, then your cat could be "dead", or it could be coated with soot and needs to be cleaned like I explained above (or something else). The rear O2 sensor should just be kinda hanging out with the numbers not moving a whole lot. Could also be a bad rear O2 sensor. This was the case with me about 3 1/2 years ago and replacing that fixed it up until just recently.

 

Thanks for this plan of attack. First I will try to get information about the O2 sensor, but I need to get my hands on a scanner (O'reilly's gave me the P0420 code). I am a little queasy about pouring any more liquids into the car right now - not too lucky with fluid additions lately. If the O2 sensor seems ok I will take that next step, carefully.

 

If a new sensor doesnt fix it, you may need to clean your cat or you could try placing a spacer/anti fouler on the downstream sensor.

 

 

Yep, the laser thermometer test was another thing I used to diagnose both my cat and my DIL's cat. My cat just no longer performing (scan tool proving that) and my DIL's cat was all plugged up and needed replacing.

 

Any luck with the spacer on the rear O2 sensor? I’ve replaced my whole exhaust system now O2 sensors and all and I still get the P0420, I’m going to try the spacer Monday hopefully.

 

Spacer worked for me to get rid of the P0420. It's been years now (68K miles), and it's still fine. Do you mean you replaced the cat as well?

 

Spacer works to stop 420 code, however, it may trip a p0136 low voltage code, depends on the length of the spacer.

 

Or P0138 as the end of the sensor moves further away from the exhaust stream. I have used a spacer before and got a P0137 o2 voltage low. Even tried putting a 1k ohm resistor in and that failed. Supposedly it works well this way as it mirrors the front o2 sensor. But I don’t know for a permanent fix.

 

My state has annual inspections. So I do not think I can use a spacer for the po420 code. If I could I would of

 

I'm in a CARB state and they do lift the vehicle at every annual inspection. Not sure if they are looking to see if people are using spacers on the O2 sensor. Even if they saw it, I can't imagine they would do more than just fail the inspection.

 

Bank 1 sensor or Bank 2

There is only one bank on this engine because it's an inline engine. I think what you mean to ask if it's sensor 1 (upstream) or sensor 2 (downstream). I put the spacer on the downstream (sensor 2 after the cat). I've never heard of putting a spacer on sensor 1 (upstream). That's not a thing as far as I know.

My state has annual inspections. So I do not think I can use a spacer for the po420 code. If I could I would of

Depends who inspects it. Some inspectors are lazy or don't care and don't want to bother looking under the car. Where I live we don't have inspections for that anyway. They only inspect to make sure all the lights and turn signals and brakes work.

 

The 90’s civic owners used to get flagged often. Because back then, they were doing obdi to obd0 harness swaps and some jdm swaps. Back then, the emissions were more lax.

When OBDII came along, California increased it even more. What some people don’t know, the load test that California performs on pre 1996 vehicle is that it really isn’t a valid test for emissions. Meaning, how many people do you know can hold 2500 rpm. Low rpm testing doesn’t emit as much compared to wot.

California has T for tamper, M for missing and I think it’s V for modified.

Tamper is when it’s been disconnected or defeated as an emission device.

Missing is more if the manufacturer didn’t equip it from factory. This is for federal built vehicles. Don’t know if these still are manufactured.

Modified is when it was a deliberate modification. Cat converters were the big ones here.

Nevada, New York and of course California use California’s emission testing or some form of it. These are the only ones I know of. California is weird because as I recall, it is evap that can be not ready. For other states, it’s any one can be not ready.

I’m sure in the next revision, they won’t allow any monitors to not be ready.