P0420 Diag

[beetles1900]

so, I had 0420 for over a year, finally got a mechanic to replace the forward sensor, and 0420 has stopped. Now I have knocing, pinging, code 0171B code 0174B. Put in a new MAF... now car won't idle. got a guess what my next problem might be?

[cardoc]

Originally Posted by beetles1900

so, I had 0420 for over a year, finally got a mechanic to replace the forward sensor, and 0420 has stopped. Now I have knocing, pinging, code 0171B code 0174B. Put in a new MAF... now car won't idle. got a guess what my next problem might be?

For starters, fuel filter or pump. Pinging can be due to lack of fuel delivery. Both of these codes indicate lean conditions or lack of fuel.

From there, the coolant temperature sensor or thermostat, but this is an outside chance. The ECT does have something to do with fuel control but mainly on the trim calculation end of it. You would need a scanner to see the data of the ECT, AF sensor (the front O2) and Long Term fuel trim to see what is going on. I doubt its the ECT. If the AF is actually stuck on lean, low voltage feedback to the ECM, then the ECM adds to the fuel injector cycle to compensate, but if it can't because of poor pressure or flow from the fuel pump or clogged fuel filter, you get the pinging and knocking with lean codes 171/174.

On the outside chance you got a defective MAF, it could be registering lower than normal air flow into the engine, which would cause the ECM to calculate fuel delivery to a leaner condition to get 14.7:1 equation, then the AF sensor would still feed back lean causing the codes.

[cardoc]

I got one in this morning that is a perfect example of what you will see in data streem that confirms a bad catalytic converter.

1996 Subaru Legacy AWD wagon, manual transmission. P0420 was the only code stored. After getting the car up to temperature and varifying that the thermostat was working properly, fuel trim was where it needed to be, and all else checked out good, the wave form from the O2 sensor shows a bad CAT.

Whenever you see a waveform as seen in the video, it is a dead giveaway. When the two sensors "mirror" each other the CAT is not reburning the HCs.

P0420 96 Legacy Brighton AWD Manual Transmission - YouTube

P0420 96 Legacy Brighton AWD Manual Transmission - YouTube

[plain OM]

cardoc:

You're making it clear, and doing it right. This thread is going to revolutionize treatment for P0420. Thanks!!

Question:

On my 07, and I presume most if not all 3rd generation, the front sensor is AF, and the ECM diagnostic output for it is "lambda", which is nominally 1.00. Would real time tracking of the front AF lambda value and rear O2 sensor Voltage (as in the last video) present the same graphing lines if the cat is bad?

[cardoc]

Originally Posted by plain OM

cardoc:

You're making it clear, and doing it right. This thread is going to revolutionize treatment for P0420. Thanks!!

Question:

On my 07, and I presume most if not all 3rd generation, the front sensor is AF, and the ECM diagnostic output for it is "lambda", which is nominally 1.00. Would real time tracking of the front AF lambda value and rear O2 sensor Voltage (as in the last video) present the same graphing lines if the cat is bad?

When the ECM is utilizing a LAMBDA measurement, it is measuring the oxygen content in the exhaust exiting the engine by averaging. The ECM is still curving the fuel trim from rich to lean to control the temperature if the CAT. The oxygen sensor behind the CAT still functions the same as far as communicating the content of the exhaust after the CAT. You still look for oddities in the waveform based on the fluxauction in STFT or irregularly high indicating a richer mix or low indicating too lean or restriction.

When the values are rich, look into ignition or temperature. When its low, vacuum leak, fuel delivery, restriction, MAF and other sensor value anomalies.

It is easy to guess at what is causing the CAT to operate the way it is when the code pops up. But we know what guessing does. A lot of techs will replace a CAT unnecessarily and the "problem" will be gone for a while because the new CAT will mask the issue that caused the failure of the original and eventually will rear its ugly head. Then you have to start over. And you can't blame a shop and win unless you can prove they new of other issues.

It takes more time to actually diagnose the issue creating the problem and I've seen plenty take the short road either because of laziness, stupidity or just ignorance. In most cases the actual necessary repair is more cost efficient than just blindly replacing the CAT. And tends to correct a lot of other issues you didn't even know about.



.

[plain OM]

Great explanation, thanks.

[cardoc]

Here is the follow up data on the 1996 Legacy Wagon 2.2 after the CAT installation. No sensor replacement was necessary.

1996 Subaru Legacy Data After CAT Install - YouTube

1996 Subaru Legacy Data After CAT Install - YouTube

[1 Lucky Texan]

was that cat blocked with carbon, destroyed by coolant or ????

[cardoc]

No. It just lost its ability to store the oxygen needed to heat up properly and reburn the HC's. This car is actually the one I put together for one of my customers, the one that has the two tone green engine. I didn't change the CAT prior because it tested good. This car will peel out. It peeled out with the bad CAT. It could still breath.

It is just an example and is only the 3rd Subaru I have put a CAT on it 3 years.

[Mtn_Biker]

I've had the P0420 code for well over a year, maybe close to 2. No other codes. I did have a new Passenger side cat welded on to get through the stupid emmisions test. I can't find any faulty performance while driving. No idle problems, overheating, etc. I admit I'm not the biggest car guy int he world. I know enough to be dangerous. I change my oil/air filter every 5-6K. She's an 04 with 175K and runs great.

After reading this, I would say that since it's been so long, the cats are probably now toast. So I need to go through this progression to find the problem...or give this info to my mechanic and have him diagnose it. Or better yet, I take a road trip to Austin to see you.

Thanks for the info. Unfortunately, it looks like I know what I need to do over the Turkey Day weekend. (After some much needed bike riding of course.)