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Old 06-30-2015, 09:02 PM   #1
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P0420 hell.

Hi, everyone. I have a 2012 Focus Titanium. I also have a scan tool with live data.

P0420 reared its head and I'm having difficulty diagnosing the issue.

I checked it with my scanner and another scanner that displays live data. Same result with each attempt.

Here is what I'm observing while at operating temp, closed-loop, and 2500-3000rpm:

Catalyst Temp = 900-1200*F (normal, as I understand it)

Sensor 2 (post-catalyst) = consistent 0.700-0.750V with spikes up and down that correspond with rich/lean conditions; blipping the throttle, for instance (this also appears to be normal)

Long Term Fuel Trim = 4% (normal)

Sensor 1 (pre-catalyst) = My Focus will only give data for Lambda and current in milliamps; no voltage! WTF is the deal with this? How am I supposed to diagnose with only current and not voltage??? The mA reading from this sensor is approx -0.500mA (Lambda = 0.900) to 2.000mA (Lambda = 2.000). Typical (average) mA reading is approx -0.050mA. This doesn't appear to be useful data - only a direct reading of Lambda.


So I thought that perhaps the #1 sensor was toast. I replaced it. No change. Same data.

The vehicle occasionally has a rougher-than-normal idle and seems to be a little more "grabby" when the clutch engages and disengages.

I am at a loss, here. NAPA is willing to let me return the replacement upstream sensor. The only other thing that I can think of is replacing the downstream sensor because...shouldn't it be reading voltage below 0.700V?

Thanks in advance


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Old 07-01-2015, 12:12 AM   #2
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Nada?
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Old 07-01-2015, 03:11 AM   #3
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Scan tools aren't helping much with the data you're getting, secondary O2 needs to show reduced cycling (spikes) compared with the primary to show the CAT is working. Code claims it isn't showing enough difference.

Out of Emissions warranty? 80k unless in CA AFAIK.
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Old 07-01-2015, 07:39 AM   #4
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Replace it with a header and a tune? :)
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Old 07-01-2015, 10:51 AM   #5
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Forgive me if my limited car knowledge is wrong here, but could it be the mass air flow sensor? Had that error on my old car once and it ended up being the MAF sensor.
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Old 07-01-2015, 11:25 AM   #6
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72,000 miles on the car. Didn't realize that the emissions warranty is 80,000mi. Is MAF covered under the emissions warranty? If I'm not mistaken, it could be the MAF.

I'm tempted to replace the downstream sensor to see if it starts reading at 0.500V or less. I'll test it later.
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Old 07-01-2015, 12:34 PM   #7
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You can see DTC descriptions here:DTC Codes (Detailed Descriptions)

MAF isn't on that listing of CAT efficiency code possible causes.

Hit up the Dealer since you're still covered, saw a couple that had CAT issues fixed under warranty with lower mileage on the car (sheet happens, usually last longer).
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Old 07-01-2015, 02:05 PM   #8
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Quote:
Originally Posted by sailor View Post
You can see DTC descriptions here:DTC Codes (Detailed Descriptions)

MAF isn't on that listing of CAT efficiency code possible causes.

Hit up the Dealer since you're still covered, saw a couple that had CAT issues fixed under warranty with lower mileage on the car (sheet happens, usually last longer).
In my case it was causing my car to run too rich in some cases and too lean in others. But the 0420 came up..
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Old 07-01-2015, 02:16 PM   #9
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Wouldn't doubt it's possible when you look at the list there of other possible contributing causes to poor operation that can affect the CAT & O2 readings.

Just not high on the list of items to suspect and other symptoms should be present if that was causing trouble.

Proves once again that "codes" don't always point straight at the cause, with more investigation often needed to prove the actual problem. Very useful hints, not cut & dried answers.
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Old 07-01-2015, 05:49 PM   #10
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Tested the downstream sensor. Seems to check-out. If it was displaying a falsely elevated O2 reading, then it shouldn't go down to 0.050V with an induced lean condition (vacuum leak). It did.

Appointment made with dealer for tomorrow AM
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