Good video, making the case for taking the time to diagnose before changing parts, in this instance through scanner logging and graphing and then (perhaps even more important) being able to interpret the information that is being provided by the OBD system.

In the video, around 0:13, the temperature gauge appears to be mid-way. I'm not familiar with the "normal" range on the 2001 and was wondering if that indication is above normal, thereby reflecting the high 230 degrees indicated on the scanner, or is this a case where the temperature gauge did not show what was actually happening?

The connection between the fuel trim and fuel filter is interesting. The LT Fuel Trim on both banks remains negative (2:29), whereas the Short Term Fuel Trim on Bank 2 is noticeably positive (around 2:47). Any ideas as to why would it be positive on Bank 2 but not Bank 1 (presuming that the fuel filter is inhibiting fuel flow)?

 

should be very helpful, can't wait to view.

subscribing for the awesomeness!

 

The temperature gauges are a funny thing. It will meander around midway until the temp starts above 215. In the video shot, the temp gauge was appropriate for the temperature, midway around 195-200.

Attached are photos I took of the scan tool screen after we replaced the thermostat, (with a Subaru part), fuel filter, oil change with 5/30 and a fresh set of NGK 6 plugs. The customer decided to go ahead and keep it. Quicker than a video upload.

The temperature fluctuates normally as you can see. The rear O2 sensor values changed to reflect the CAT is working as it should. No more dumping the oxygen. The fuel trim changed 4% over previous toward the positive also indicating a good combustion. The photo are at idle, cruise rpm then idle again.

Final result is no catalytic converter, no AF sensors, no O2 sensor, no other sensors. The ignition plugs that were removed had worn to an average gap of .046. Still manageable, but worn none the less. Replaced as a preventive maintenance.

 

Awesome Cardoc!! I had my car throw that code and my mechanic said the same thing. New CC!! I had him reset the lights and they still are not back on. No new CC for me. I do believe mine was bad gas. I filled it up with new gas and the light has not come back on.

 

that doesn't mean too much - pretty good chance of the code coming back in a month or a year. hopefully it doesn't but it would be good for you to know that it's fairly common for these codes to make guest appearances that increase over time.

 

I have seen a number of vehicles come in with the 420 code and it ended up being the gasoline. A lot this time of year when the oil companies are switching to winter blends. Mostly through the year it is, in decending order of repairs, thermostat, plugs (or other ignition), filters, exhaust or intake leaks, injector seals, injector(s), MAF/MAP sensors, rear O2, AF sensors on Japanese cars, front O2 on others, and lastly the CAT.

I do get some that require a new catalytic converter, but that is only after long periods of running a vehicle with issues whether it be the type of gas, ignition plugs worn, vacuum leaks, clogged fuel filter, etc., and they are few and far between.

Catlytic converters are designed to last a great number of years as long as the engine performs as it was designed. Too rich or too lean kills them quick whether with carbon build up or overheating. If a catalytic converter is bad, you need to make sure its bad then find what caused it to fail. Otherwise, you will destroy the new one.

And, some shops are quick to replace a CAT because they are actually cheaper than you think when purchased in bulk wholesale or jobber and the markup used is outrageous, then you have the labor added on which comes down to profit margin. A new cat will burn more efficiently than a used one and will mask a problem with engine performance. These shops know this. Some maybe not and they are under the impression that a 420 code is a dead giveaway.

If that engine light comes back on for P0420, Catalyst Efficiency Below Threshold, go through the list and check everything on it first.

When I get a vehicle in with a bad CAT, which may take a while, I will post a video of the data so you can see the difference in the feedback to the ECM and you will know what I am talking about. And, the sensor data is available on a lot of the low cost scanners you can buy at a parts store.

 

This has been a really very informative thread and I look forward to more of your ideas for diagnosing problems.

cardoc: I have a general question that comes out if the video. Do you find that having a car in the shop idling or manually revved up is usually sufficient to arrive at a diagnosis through graphing/logging of parameters, as in the case of the video, or do you find that often it's necessary to go out on the road while observing the scanner (with someone else driving, of course)?

The genisys EVO you're using is quite the scanner (referring to showing several graphs at the same time).

 

Depends on what I am looking for. If its transmission related or VVT, then I need to drive the car and I record while I'm driving or glance over at the graph to see if there is a change I'm looking for.

Usually, unless it is something directly related to engine load, I can do it in the bay.

And the EVO will print the recorded data to my printer for visual aid to the customers or for ease of looking at the data stream without toggling up and down the screen.

 

Thanks for the helpful reply.

And the EVO will print the recorded data to my printer for visual aid to the customers . . .

Having the right tools, and knowing how to use them, shows it's worth.

I had a look at the EVO website and was impressed by the genisys EVO; the unit is understandably professional-level, but quite comprehensive and beneficial.

Will look forward to more valuable insights such as this one on P0420.

 

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?

 

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.

 

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.

 

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.



.

 

Great explanation, thanks.

 

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

 

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

 

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.

 

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.)

 

I had another Subie in today with a 420 code. Ended up being the fuel pump. Too weak to supply sufficient fuel and pressure. It was 12 psi short when I tested it. Fuel filter was good. He also had a long stretch of cheap gas. I forewarned him that he may be looking at a replacement CAT later on. Maybe not. I advised him to change his fuel brand and run premium through the winter months to help compensate for winter blends and we aren't cold enough.

I took a video of the scan tool data but I inadvertently deleted it from the phone. But to give you a run down, LTFT was at 11.7, the rear O2 was high indicating rich. The plugs were the wrong ones, so I went ahead and replaced them with platinum NGK. In the end, fuel trim dropped to 2, rear O2 dropped to avg between .55 and .65. I was also reminded why I opted out of owning a Forester. You can't just tilt the seat up to get to the harness, and too much to remove from the rear to get to the fuel pump. But it all worked out good.

 

cardoc:

Any experience with the 2005+ and perhaps other later models?

There doesn't appear to be a specifically-identified "STFT" and "LTFT". (See attached list from 07 FSM).

I believe the relevant PIDs are "AF Correction #1" and "AF Learning #1" respectively but would appreciate your comment, especially as this might be useful for others using this thread as a P0420 troubleshooting guide with a later model that might not read out the STFT and LTFT.

Also note that there's also "AF Correction #3" and AF Learning #3" which, like the other two, will show up with those descriptions in scanner readouts but are not otherwise identified as to their significance.

(Incidentally, all of these PIDs show up when scanning my 07 engine PIDs with Romraider Logger, however "Intake Manifold Absolute Pressure" is identified as "Boost". It took me a while to make this connection . . .)

 

For some reason, your attachment isn't opening. I'll look into that later.

RomRaider is a tricky software package. The definitions for the ECMs are generalized for use with multiple types of ECMs and TCMs.

The Denso ECM begins a learning curve for fuel trim every time its started. It does this due to changes in temperature, pressure, etc.. Once it learns the new fuel trim data necessary, it stores it and uses that value for all closed loop operation.

The AF values are calculated in Lamda, EQRatio 1/1, 2/1, 3/1 depending on H4, H6, NZEV or PZEV vehicles, but you can change the value in RomRaider to show AF at stoichiometric if it makes reading the data easier.

The AF correction and AF learning are the values utilized by the ECM to calculate closed loop trims. The LTFT and STFT can be calculated based on the Learning and Correction respectively. You just have to watch it while you log and do the math. Or graph it and watch the waveform. Once the computer realizes there has been a change in data input, it goes back to the correction and learning to compensate. Its when there is an issue with performance and the computer is making a lot of correction in its learning curve that the MIL comes on. It sees an issue with the LTFT learning, or that input data is out of range and sets the code for the system it believes is the issue.

Now, to effectively diagnose the issue with a P0420 using aftermarket software and determine where the problem lies, and we will use RomRaider since it is widely used by Subaru owners for various reasons, log the data from the rear O2 sensor. In RomRaider, click the Rear O2 Sensor and use either Amps or Volts. When graphing this sensor, the activity of the sensor is the same as any other scan tool and the value will remain steady rich or lean indicating a fuel or ignition issue, or it will fluctuate rich/lean based on the AF Learning value

You don't necessarily need the AF values to determine if a CAT is bad. All you need to remember is the O2 sensor reports the Oxygen content of the exhaust exiting the CAT. Anytime you see a wave instead of a steady value, the CAT is bad. Otherwise, with a 420 code, you are looking to see if the O2 is rich or lean, then start with locating the issue with engine performance following with sensor outputs.

As far as the RomRaider component title in the log lists, as long as you know what you are recording, all is good. As long as you understand what values are necessary for proper performance.

My VDC actually reports Lamda values, but I can alter the RomRaider to show Stoich. I also have different scan tools that show different values. The EVO will show Stoich or Lambda depending on which value I choose to look at, either AF or EQ.

I will record a video of my car and post it if I get time today. I will use both the EVO and RomRaider. You'll just have to remember I am running E85 and have 2 6.2L cats installed so the O2 sensor values will be scewed. I just get hot air out the tailpipe.

 

Cardoc, need some of your wisdom on gas and the P0420 code. Reference the original P0420 problem I had here.

Since fixing the o-ring on the IAT/MAP sensor I have not gotten the P0113 code, but I have gotten the P0420 a couple times now.

When I was in Germany all fuel had 5% ethanol and if I remember correctly I was using 95 (RON)...so probably around 90-91 in US standards (AKI). I never got the P0420 code in the three years I drove over there. First day back in the states, first tank of gas, I get the code. I've been using mid-grade here in NC. I've gotten the code within 50-60 miles of each refuel the past few weeks. The last refuel, I put in premium, no code yet.

Ideas?

 

The brand of gasoline sometimes does make a difference in P0420 issues. Murphy gasolines (Wal Mart) and certain grocer stations purchase what is referred to as "shelf" blends. I have customers that have helped me out with experimentation with gasolines by running a tier 1 gas in the car then switch to the cheap stuff. After filling with the cheaper gas, the MIL would come on for 420. We would leave it alone through the tank of fuel and they would refill with the tier 1 gas and after driving, the light would go off.

My VDC does it now. I run E85, no MIL. I go back to Premium and the light will come on for too rich. Leave it, go back to E85, and the light goes out after 10 miles driving. This car is programmed for boost and fuel and it doesn't like gasoline any longer. The last few reflashes we did to the ROM was on E85, so the injector cycle makes it run rich with gas.

Low quality fuel does not burn well and the high HC in the exhaust reduces the oxygen which causes either a rich code or 420.

 

 

cardoc:

Once again, thanks for the detailed answer/explanation in post #24, and the video. It's a real lesson for me, and hopefully will help others.

I will look back at Romraider Logger to see if I can switch out of the Lamda indication -- I hadn't noticed that, but didn't pay attention because this is the readout term the FSM indicates.

But it's your explanation of the relationships that's going to be most useful. And as programs such as Romraider Logger, and FreeSSM, are easily available, and need only a connecting cable, readers should be much better able to deal with the P0420. (For others not familiar with FreeSSM and Romraider Logger, see: http://www.subaruoutback.org/forums...5-parts-accessories-performance/39426-freessm-complete-access-your-ecm-tcu.html)

Edit: Re second para. above, cardoc is correct; I checked and Romraider can be changed to read sensor detail rather than Lamda.

 

p0420 ,2005 outback

got the classic p0420 code... I believe this is under a recall from suburu does anybody know?

 

There is a "recall" for reprogramming of the engine control module software. This is supposed to help avoid future failure of the catalytic converter. Part of the reprogramming procedure involves tests to see if the catalytic converter is already failing, in which case it would be replaced, otherwise the software is just updated. However, as this thread demonstrates, a P0420 does not necessarily mean the converter is faulty -- the code can be caused by something totally different, such as the thermostat.

 

And, when you take it in to the dealer, if they find something other than what is outlined in the recall that will correct the efficiency of the CAT, if its not warranted, you pay.

How many miles? Filters good? Fuel cheap or good brand?

 

Cardoc: thanks for the explanation and vid. Wish I had a mechanic like you in my area. I had the P0420 come on again, 2 miles after filling with a different brand of gas while on a road trip this weekend. So far only the gas sold on base hasn't caused the code to come on. Maybe that's the key, not sure. Kinda scratching my head.

Anyways, I had my scanner and got the following off of it:

Codes(2):

P0420 Cnf ModE1
Catalytic System Efficiency Below Threshold Bank1

P0420 Pnd ModE1
Catalytic System Efficiency Below Threshold Bank1

Snapshot data (in its entirety):

ENG SPEED: 2300rpm
CALC LOAD: 18.8%
MAP: 59.0("HG)
COOLANT: 83(F)
ST FTRM 1: 7.8%
LT FTRM 1: 0.0%
VEH SPEED: 94 (I was going 65 when it came on, so not sure why this is so far off)
FUEL SYS 1: CLOSED
FUEL SYS 2: N/A

 

That's a snapshot of the freeze frame at the time the problem was first "seen" by the computer. Depending on how long the engine was running when this occurred, I see the temp is low. Within 8 minutes the engine temperature should be up to operating temp.

The ECM turned the light on after repeated occurrence of the fault. The MAP pressure is high, so I can assume its not a turbo running high boost, you may want to look in to that.

So if your scanner shows live data you need to run the car from dead cold and look at the temperature, MAP, front AF and rear O2 sensor output. The temperature needs to get up to 190-200 and average in that range. Your car runs AF sensors in the front, so you will be looking for the same values as I have in the VDC scan video and midrange to low O2 values. If your engine temp is remaining low, change the thermostat, then while you are verifying proper function and engine temp, watch the sensor readings for a change.

You have the advantage in owning a scan tool. The freeze frame is good. It's better if it reads and outputs data.

Also, the short term trim is high, but with the temp and speed of the car, that would be normal. If your temp was up, it wouldn't be that high.