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The 411 on a P0420

Thursday, August 1, 2019 - 06:00
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My friend, who happens to be a fellow amateur radio operator, sends me a text message with a picture of a scan that has been performed on his 2012 Ford Fusion and asked, "Do you have a few minutes to help me find out the problem with my car?" He goes on to say, "Check engine light is on for a P0420. I'm sure it's just O2 sensors."

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I'm usually cautious about volunteering information over the phone about what could be wrong about a vehicle when I haven't had my eyes, ears, or at least some scan data from the vehicle. I get even more cautious when my ham buddies, who are a lot smarter than I, ask me for input.

The subject vehicle is a 2012 Ford Fusion with 87,000 miles and an automatic transmission. The MIL is on, it's running rough and has poor performance.

I don't have the confidence that I should when it comes to automotive diagnostics. I've been wrong more times than I can count and I don't want to be stuck with a nightmare I can't fix — especially when it's on a friend's car and when it's a "routine" P0420. When I look at catalyst efficiency codes nine out of ten times the ECM identifies a faulty catalyst. The ECM may hint at the reason why it failed with other DTCs but most of the time. We're left to find the reason why the cat failed on our own.

Sometimes we get unlucky enough that the replacement converter doesn’t fix the catalyst code. That is going to be a bad day for everyone involved.

OK, let's go for it!

There are five basic quick checks that I want to perform when I see a P0420. Those quick checks include identifying any current misfires or misfire history, identifying any fuel trim problems, validating the downstream oxygen sensor voltage, confirming that the downstream oxygen sensor is switching and searching my service information for software updates and/or common problems.

So I ask my friend, “Do you think this car has been misfiring before the MIL came on?" He believed his vehicle didn't start running bad until after the MIL came on and it's not always performing badly. It's an intermittent performance issue. That's backward from every other situation I've ever seen. Usually, it's running bad and then the MIL comes on with a catalyst efficiency code.

An auto scan of the vehicle confirmed that the only codes present in the ECM are a P0420 catalyst code and a p1000. For those of you who haven't seen a p1000 on a Ford product, it means the ECM has not completed all of its monitors. This is a really good hint that someone has been clearing codes before you worked on it. If he cleared the codes, he also erased any potential misfire history and Freeze Frame data that may have been stored. So at this point, I have to trust that if I cannot duplicate a misfire on my test drive or in the bay it didn't have a misfire. At this point in time, if the cat does prove to be faulty, I don't think a misfire is the underlying cause.

The next of my quick checks is to identify any fuel trim problems. Fuel trims problems are a bit subjective. What I mean by that is some technicians or instructors say that plus or minus 10 percent is ok, others say plus or minus 5 percent. When I have analyzed newer cars without problems, they are closer to plus or minus 5 percent in my experience.

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