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Old 02-03-2012, 09:44 AM   #11
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Both sensors are working properly but yes, the switching downsteam sensor indicates that your cat doesn't have the ability to store oxygen very well. If this where another vehicle I'd say your cat is wasted and should be replaced.

However, from my experience this is quite normal. The cats on the SVT aren't very efficient and the ONLY time I've ever seen a steady downstream signal on one of these was at about 3000rpm down the highway. I'm guessing the VCT system does a good job of controlling NOx so HC and CO are the only post-combustion emissions the cat needs to deal with. Naturally, a well-tuned mild 2.0L isn't going to have very high HC and CO levels so the emissions threashold required to set the P0420 DTC is rather generous. In other words, eventhough the downstream signal switches like crazy, the emissions are low enough that the government doesn't require a code to be set.

But I wouldn't be surprised if your code comes back. Eventhough your signals look like other SVT's, your PCM obviously saw an excessive switchrate so you either had a momentary fuel quality issue or your cat is loosing it's ability to store what little oxygen it's inteded to store...
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Old 02-03-2012, 11:51 AM   #12
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Another thing I wonder about is that all the heat shields have been discarded from underneath, is it possible that at cruise throttle, the pipes are cooling off enough to not let the converter burn? After a run, stop and idle and the rear oxygen sensor drops to a flat line, the above graph was part throttle cruise. Normally on my commute I'm running about 1800 rpm 40 or so in 6th gear.
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Old 02-03-2012, 01:07 PM   #13
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Another thing I wonder about is that all the heat shields have been discarded from underneath, is it possible that at cruise throttle, the pipes are cooling off enough to not let the converter burn? After a run, stop and idle and the rear oxygen sensor drops to a flat line, the above graph was part throttle cruise. Normally on my commute I'm running about 1800 rpm 40 or so in 6th gear.
Meh. The shield is more to keep the heat away from grass and underbody than it is to keep heat in. Sounds like the cat is capable of storing oxy based on your post-cruise datalog.

How many miles?

It may be on it's way out but proper driving cycles and fuel quality will prevent the P0420 from repeating.

Also... I've noticed some non-oxy 91 around here. I didn't notice much of a difference but the switch rate threshold for your downstream 02 is calibrated for oxygenated fuel. It's possible that a batch of non-oxy on a worn cat could make that downstream switch high enough for the PCM to set an efficiency code.
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Old 02-03-2012, 01:34 PM   #14
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Right around 95,000 miles so far, seems the previous wrench discarded everything heat shield related. I've been getting brand gas from two stations (Mobil) not the econo place, I suppose a newer front O2 sensor could give a bigger split in the signals?
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Old 02-03-2012, 01:37 PM   #15
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Perhaps but the cat monitor isn't really comparing the front to the rear so much as just focusing on the rear during closed loop. Obviously it isn't looking closely in OL drive modes like WOT or extended idle but I've never heard of a bad (or brand new) front 02 erroneously setting cat efficiency codes.

If the light doesn't come back on I wouldn't worry much.
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Old 02-03-2012, 01:39 PM   #16
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Yeah, P0420 showed up in 'pending' drat, maybe not letting it idle enough, usually start and go :) Suppose with a harness plug from the boneyard and the old O2 sensor plug part could make the resistor adapter, though willing to try and make it right.
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Old 02-03-2012, 03:53 PM   #17
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Yeah, P0420 showed up in 'pending' drat, maybe not letting it idle enough, usually start and go :) Suppose with a harness plug from the boneyard and the old O2 sensor plug part could make the resistor adapter, though willing to try and make it right.
Yeah if you can't tolerate the CEL I'd consider a cheater/emulator. The only cat failure that will affect performance is melting and that's a completely different scenario than inefficiency.
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Old 02-04-2012, 07:44 AM   #18
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Oddly enough, planning to pull the front o2 sensor and it has another code pending, #1 o2 sensor not switching enough, too lean. I pull it out and its pretty darned black, the old rear one was light tan, wondering if its been sending bad data or if I have a misfire that's not being caught.
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Old 02-04-2012, 09:34 AM   #19
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Let's see that graph after you replace bank 1.
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Old 02-06-2012, 10:17 PM   #20
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Well replaced the front 02 sensor, it was about 90% jet carbon black. Also changed the pcv valve and inspected the rubber hoses, all seemed fine. Here is another strip chart, front sensor voltage is indeed a bit higher, scale is 0 to 1 volt.



Is the Focus SVT converter really 2 parts? It looks like Header, o2 sensor, oval cat segment, rear o2 sensor, then another bendy oval cat section before it bends up and around the axle.

I have seen the P0420 come and go once since
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