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fast*st 02-02-2012 08:13 AM

oxygen sensor graph review help. Cat dead?
So managed to graph oxygen sensor output on the Focus SVT, cleaned the MAF, changed the PCV and trying to track down a P0420.

Red is front, green is rear, it seems like the voltage is lower on
the rear but not sure if its enough. On the standard xl graph
view for O2 sensor voltage, the rear is graphed on an 11 volt
scale and the front is a 1 volt scale so the rear looks smooth.

Any input appreciated!!


fast*st 02-02-2012 11:27 AM

Not sure if its relevant, did reset the MIL and the system isn't ready for testing yet, only 20 miles since reset.

Mike_Mac 02-02-2012 12:49 PM

P0420 is for a failed/failing cat or bad rear o2

the rear o2 should be very close to a straight line (if i remember correctly)

6SPD_soul 02-02-2012 05:49 PM


Originally Posted by Mike_Mac (Post 3993238)
P0420 is for a failed/failing cat or bad rear o2

the rear o2 should be very close to a straight line (if i remember correctly)

Yeah, that's correct. I don't really understand some of the markings on the graph but those should both be between 0-1V. The front should fluctuate fairly quickly between 0-1V and average about .45 to .5V. The rear should be a basically straight line, it's mostly there to show the PCM that the front is maintaining the correct average.

The fact that you're getting a signal means the wiring is okay, I'd just replace the rear sensor, clear any codes, and retest it however you did that graph. How did you do that graph anyway?

1turbofocus 02-02-2012 07:16 PM

Both look about right and they are switching


fast*st 02-02-2012 08:26 PM

Thanks, the graph is from scanxl pro the voltage is 0 volts at the bottom, 1 volt at the top. The rear 02 sensor had failed and that is the new one, old one had o2 heater failure and no output. So there seems to be 2 schools, on a full dual cat system the rear is a flatline and the front is the cyclic pattern. On the smaller single cat they can both oscillate with the rear being of lower amplitude from the front. I'm just not sure what the correct SVT pattern is. Red is the front sensor, yellow is the rear.

6SPD_soul 02-02-2012 08:50 PM

I guess I was wrong about how the o2 sensors work on the SVT, a lot of other cars with a dual bed (single housing) cat will have a front sensor with that swinging voltage wave and a rear sensor with more of a flatline.

So the graph shows a reading of the new rear sensor, and you don't have a CEL on again yet right? I got sidetracked by the fluctuations of the rear sensor and skipped the part about you driving 20 miles without a new CEL. If that's the case it's fixed and operating correctly, those are monitored continuously as soon as they are hot enough, after a minute or two of running and relaxed driving.

fast*st 02-02-2012 09:32 PM

Well, there are two different flavors of cat and oxygen sensor setups, I'm not sure which one the SVT is supposed to have. So yeah, new rear sensor shows a slightly lower swing than the front. I don't have the CEL yet but well, I'm not sure what it looked like last time it threw the code. The last time it took about 55-60 miles before it came up to pending and then came on ten miles later.

whynotthinkwhynot 02-03-2012 05:37 AM


Originally Posted by fast*st (Post 3993891)
So there seems to be 2 schools, on a full dual cat system the rear is a flatline and the front is the cyclic pattern.

I believe what you're referring to is the difference between narrow and wide band sensors. Some manufacturers use wide band sensors which display continuous readings- flat line- if you prefer. Typically those are used in conjunction with cheaper narrow band sensors resulting in one jumpy line, and one flat line on those readouts.

Those look fine dude, like Tom said. IMO your bank 1 sensor needs to be replaced since it's reading lower than the new rear sensor. It's working, but it would put out more voltage if it was new. You'll probably pick up 2 mpg with a new front sensor.

fast*st 02-03-2012 08:30 AM

Well, its worth a shot before doing anything drastic, thanks for looking it over! The rear sensor had zero output before I started troubleshooting, that was replaced, the front is original with 95k miles on it. Hopefully the cat is happy. Oddly enough, after a drive to warm it all up, at idle, the rear sensor smooths out quite a bit but not while driving.

So O2S11 (sensor 1 front?) is just a tick higher in voltage from O2S12 (sensor 2 rear?) A newer front sensor would increase the voltage and put it closer to happyland? its an easy parts swap so I'll try to knock that off today. Today shows a P0420 in pending status

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