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Discussion Starter #1
I've searched this forum about the P0420 code. Subaru calls it "Catalytic Emmissions System Operating Below Peak Efficiency" or similar language.
This problem...I'm told... is usually an O2 sensor problem.

The car is an 02 Subaru Outback Wagon with about 150,000 miles. The first time the code came on was around 142000 miles. I finally replaced the rear O2 sensor and the code went away for several months. It has recently re-appeared. Car appears to run fine. So I decide to capture some live data with my code reader...Innova 3140b. I attached a screen capture of the computer screen showing the engine rpm, voltage on sensor 1 and the voltage on sensor 2.

If anyone has any experience with this please comment. Are the voltage fluctuations on sensor 2 normal? Does the voltage on sensor 1 look normal?

Thanks!

Jim
 

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The car determines catalytic efficiency by comparing the primary and secondary O2 sensors. People are of the belief that if the P0420 is thrown that that means there is something wrong with the secondary O2 sensor or the cat. Early OBDII systems did only rely on the secondary sensor but modern ones compare both sensor outputs to determine efficiency.

A P0420 could be a bad secondary 02, a bad primary 02, or a bad cat. If you do not know the history of your primary sensor I would replace it with a Denso or NGK.
 

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Indeed, these electronic control systems are now very complex, and often use a "fuzzy logic" approach, so there is no fixed relationships. It makes understanding data that is available more challenging.

I've been looking at the graphs, and while I don't know if they indicate a problem or not, there's some things that puzzle me and that perhaps others can explain:

The first is the engine rpm, which is oscillating between (approximately) 700 and 900 rpm and back every 20 seconds. That "hunting" should have been noticeable during the eight minutes of recording time, and doesn't seem normal.

Sensor 2, interestingly, tracks up and down with the rpm. The lower voltage coincides with the low rpm (or the drop down to it), and the high voltage with the higher rpm. Low voltage at the O2 sensor itself usually indicates a "lean" condition (lots of oxygen in the exhaust), which is consistent with throttle backed off while the engine slows down. Similarly, the 0.8 Volts at the O2 sensor is indicative of a rich condition (little or no oxygen in the exhaust), which is consistent with the throttle being advanced to get the engine revs up. I believe this is normal for the second sensor.

But, the more-or-less constant voltage at 1.5 Volts at sensor 1 isn't what I would expect; rather, I believe (and am open to correction) that in closed-loop operation, the sensor voltage should be changing rapidly (at least once per second, but usually much faster) between roughly 0.3 and 0.8 Volts.

In this regard, on my 07, the OBD II output does not supply sensor 1 Voltage; rather, the output is "Lambda", which is ideally 1.00 (stoichiometric air/fuel ratio), but could vary above and below. Could the 1.5 Volts read by the Innova actually be the Lambda but as a Voltage rather than a ratio?

If the graph does, in fact, reflect the sensor 1 Voltage, then it seems to me something is wrong. Perhaps the front O2 sensor is bad.

But there are others here with far more understanding of the sensors and how they function, so I put these ideas out only to, perhaps, start a discussion on interpreting the data.
 

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The front sensor might be "lazy", especially for an wide-band type. I fixed a P0420 code on my 1995 Impreza a few years back by replacing the front sensor; the rear one is still original.
 

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The 2001 models began the company wide compliance to California Emission standards and all models were considered ULEV.

The front O2 sensor is an A/F sensor and is measuring the oxygen coming out of the engine. The rear one tells the ECM whether the CAT is working or not. The CAT's job is to reburn the exhaust to lower harmful emissions. You have run 8k miles with the same problem or additional problem and may have damaged the CAT further. You know a CAT is bad when it matches the STFT exactly. When you see quick fluctuation with the rear sensor, the CAT is dead. Usually when a sensor fails its because its old, heater circuit failed, or damage to the harness feeding it. Also, never use a universal. They don't work long and will send the wrong info to the ECM which will then try to adjust for what it receives. Always use OE fit whether Denso, Bosch or NGK.

Several things determine the "efficiency" of the CAT. Proper ignition, temperature, timing, fuel, A/F and the actual condition of the catalyst material within the CAT.

The "hunting" for an idle generally indicates a vacuum leak. A vacuum leak will also cause a lean mix. If you can look at the LTFT, if its positive and high, the ECM is adding, negative its reducing. If its adding, find the leak or clogged fuel filter. If its reducing, ignition or fault with the MAF/MAP/BARO, whichever your engine is running. With the engine hunting, its changing the injector pulse and timing with the up/down of the idle to try to maintain stoich.

The engine temperature needs to maintain at around 195-205 in order to assist in combustion. After the ECM detects full operational temperature, it switches programming instead of enriching the fuel to warm it up faster at low temperature.

Plug condition and their ability to ignite determines the combustion. If they are weak, poor burn, higher HC's and poor CAT efficiency.

Poor quality fuels will not burn efficiently. This time of year, the fuel companies are switching to "winter blend" for colder climates mixing in with the current fuel in the station tanks. This may have something to do with it at this time, or its just related to a vacuum leak or other sensor failure.

Also, if your system is getting poor amperage conductivity, it slows the ECM's ability to run its systems. Like a laptop on a low battery, it slows down. And with the idle fluctuations, the alternator load is switching too fast for it to maintain proper output which will lead to a low output battery and alternator failure if it continues too long. Check the grounds.

So, search for a vacuum leak. Broken/cracked hose, intake seals, etc.. If its MAF managed, check that the MAF is actually operating properly and giving the ECM what it needs, dito with the MAP/BARO. Check your engine grounds and conductivity of amperage. Then move on from there.

P0420 does not necessarily mean a bad sensor or CAT. You have to check the underlying systems that effect the performance of the engine. Think of a 420 code as telling you your engine performance sucks and you need to find out why.
 

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The upstream sensor waveform doesn't seem normal (especially if there was indeed a vacuum leak). Things would look much better if the two waveforms were swapped!
 

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You can't pull freeze frame with innovate software, so you are left with hands on testing of the sensors to determine if they are indeed failing.

Check resistance between pins 2 & 5 on the AF sensor plug. (See Pic). If it is less than 10 ohms, the AF sensor is not the problem.

Did you find the vacuum leak?
 

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I took what we're seeing in the attached graphs as "live data" recorded over a six minute period with the engine idling, rather than "freeze frame" information.

Aren't pins 2 and 5 the connections to the A/F sensor heater? If so, checking for continuity is a good idea -- if the heater is open-circuit, the A/F sensor won't work, although, in this case the OBD should have picked up the faulty heater. However, is it not possible that the heater is fine while the sensor itself isn't functioning properly? (I'm still puzzled by the relatively flat line for Sensor 1.)
 

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Discussion Starter #9
Thanks to all who replied to my post.

First, as CarDoc says the 200 RPM "hunting" should have been noticeable but it was not. There was no noticeable change in the engine
sound and I was watching the Tach and it was steady. One would have to be deaf, dumb and blind to not notice that big of a change at idle. The other thing is this, after putting 10 gallons of fuel in a nearly empty tank today, I noticed that it took the fuel gauge nearly five minutes to indicate 3/4 tank. Now the fuel gauge has nothing to do with this problem, but if that gauge takes that long then maybe Suaru built in similar "smoothing" into the Tach readout? But...I should have heard the engine changing RPM.

The other negative is that I'm not sure if the engine was warmed up or not. That would make a difference. I called Innova tech support and they had no explanation for the RPM graph showing the oscillation in the idle.

Jim
 

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The AF sensor can be read 2 ways. The fluctuation in the AF sensor is related to the flux in idle. Find the issue with the idle and then check temperature, ignition, grounds, sensors output after any repairs in these areas, then you will know if its a CAT or not.

Another way you can check the sensor operation is to open up the intake tube so you can shoot a quick spray of starter fluid or other flammable into it while watching the sensor output. If the sensors switch to rich, they work. Do the opposite by creating a vacuum leak and watch for lean. And again, at operating temperature, watch what the rear O2 does. If it switches from rich to lean a lot, its a bad CAT.
 

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Discussion Starter #11
Well...the engine at idle was varying about 200 RPM every 10-20 seconds.
I've gotten so used to hearing this sound that I guess it did not even occur to me that it was happening. Yesterday, I collected more data and the idle is actually cycling 200 RPM. The reason it is doing this is because the cooling fan on the radiator is turning on that often to cool the engine while at idle. I assume the PCM then increases idle speed because of the fans turning on...but that's just a guess. This vehicle has run great and has NEVER overheated. Of course...this process does not take place when the vehicle is in motion and air is passing through the radiator.

So...my question is...has anybody else noticed this behavior at idle?
 

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Never had the fans cycling that fast on my 07.

Another cause of automatic engine rpm adjustment could be the AC compressor, and the radiator fans come on whenever the compressor is running. The ECM increases engine speed to accommodate the extra load of the compressor. So what you might be experiencing is the compressor cycling on and off, which would cause the fans to cycle at the same time, and the engine speed to adjust. Any chance during the test the HVAC AC was turned on, or the heater system was on and was set for front windshield defrost?
 
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