The 411 on a P0420

Aug. 1, 2019
When working on a friend’s car, I don’t want to let them down. It is a lot of pressure to ensure my diagnosis is accurate the first time!

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."

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.

I checked fuel trims and trims are darn near perfect plus or minus 5 percent even under load. It's important to check fuel trims during a road test in all operating ranges in order to rule out a trim problem. Like the misfire monitor, the fuel monitor is continuous and fuel trims can be off in one rpm/load range and just fine in others. I don't think a trim problem ruined the catalyst.

So I move on to my next batch of data. I start the car to warm up the cats at 2500 rpm for a few minutes and I watch the downstream 02s. The bank 1 downstream 02 is switching very rapidly, indicating the catalyst can no longer store oxygen. At this point, I agree that the cat is faulty but what caused it? I still haven't identified the performance problem either. I do know that my downstream oxygen sensor is not skewed low by an exhaust leak because it is switching from 0.1v to 0.8v. In my experience, a cat code set by an exhaust leak post cat will suck air into the exhaust and pull the downstream 02 low due to a high amount of oxygen in the exhaust.

After the MIL has been addressed I start to focus on the drivability problem. There were two big hints that led me to the path I chose next.

When I went for a drive I did a couple of WOT runs and I felt like the car had power but it just wasn't all there. The only reason I picked up on this is that my fiancé happens to have the same car which I drive frequently. This lack of power wasn't much but it was noticeable.

The second giveaway is the lack of a 1-2 shift under heavy throttle. I'm not saying I put the engine's safety in jeopardy but trust me it wouldn't upshift. I thought to myself at the time, this is going to be an expensive bill if he's got a transmission problem and bad cats. And then it hit me. I need to do a Volumetric Efficiency calculation.

Is this a breathing issue?

A VE test is used to identify breathing restrictions, or more precisely, it helps identify any issues with the engine's ability to take air in and get it out again. A volumetric efficiency test compares the total theoretical air charge an engine of a given displacement should be able to take in to what it actually takes in, and expresses it as a percentage. After all, an engine is really just a big air pump.

I recommend running the numbers several times with scan data to accurately average the data. To perform a VE test, you're going to need a safe testing area near your shop where you can safely perform a WOT pass, starting from a slow roll in first gear and continuing right up to the 1-2 upshift. You'll also want your scan tool plugged in so you can record mass air flow in grams/second, intake air temperature and engine rpm. Graphing is better than recording the data numerically. It makes it easier to pick out the numbers you'll want for the next step.

Notice the rise in intake air temperature during this VE test run. Why is the air in the intake getting hotter when it should be getting colder?

You then put these numbers into a VE calculator. Many are available online — just "Google" it. During the 1-2 shift, the engine will be at its peak breathing range. With the car I was working on, I had to manually upshift this car to avoid engine possible damage due to over revving. What I saw in the scan data was very exciting to me!

A rise in intake air temperature during acceleration! I had never seen this before…ever!

Sometime ago, probably in early 2017, John Thornton was doing a class and this topic was brought up. I'm blessed to have had the opportunity to attend his classes. The conversation I had with John about rising intake air temperature was a side note in his class. A side note! I'm telling you this blew my mind. One thing about John if you have ever had him as an instructor is he is so humble, so intelligent, so dedicated but also just so simple sometimes. When he spoke of this technique a couple of my classmates were just mind boggled. I've only been working on cars for about six years now but I've done hundreds of VE calculations.  Not once did I ever think to look at intake air temperature closely.

"On a good car," (and I'm quoting this from a text John and I shared), "I expect intake air temperature to decrease as air mass increases." How many times have I seen this and paid no attention to it? Hundreds of times I've seen this during VE testing. I never thought about if intake air temperature increased with a rising air mass that it would mean something - or be a useful diagnostic strategy!

What does it mean??

Intake air temperature drops under acceleration because the engine is breathing in lots of cool air and it cools the intake air temperature sensor and the engine. So if intake air temperature increases, we have a restriction preventing the engine from bringing in cool air. This restriction could be an intake restriction or an exhaust restriction. I suspected an exhaust path restriction due to the ECM setting a P0420 and the scan data evaluation.

Always prove the fault

I prove this another way. I've written before about testing your hypothesis two ways to fact check yourself. I’m already counting three different data points to condemn the cat. But I want to see if I can dissect this a little more. See the ECM didn't set a p0430, it only set a p0420. So I anticipated that the restriction was on the bank that set the cat code. I was wrong in that anticipation. The exhaust path restriction was notated on both banks, as you can see in my in-cylinder running captures with a pressure transducer. When I do a snap throttle test, both banks exceed 30psi on the exhaust stroke. So if my ECM is OK with my Bank 2 cat but there is a restriction noted on both banks, where is the problem?

Prove your fault with more than one test before pulling the trigger on a repair. In this case, I'm going to measure backpressure on both banks using an in-cylinder test.
Both banks built up in excess of 30 psi on my snap throttle test. Exhaust restriction confirmed!

In the rear catalytic converter. Like many V-6 and V-8 designs, there are three catalytic converters in the exhaust — one at each manifold and one downstream after the Y-pipe. The Bank 1 cat's ceramic, mounted to the rear manifold, broke apart and ended up clogging the rear converter almost completely.

The technique that John mentioned in his class really helped me with this diagnosis for one simple reason — understanding what a known good VE calculation for this engine? For a long time, an acceptable VE calculation range was 75 percent-90 percent.

The Bank 1 cat (the one with the P0420 code) had a failed ceramic that broke apart and clogged up the rear cat on its way down the exhaust path.

What I want to focus on here is when I ran the numbers and averaged my VE calculations I was coming up with about 75 percent. Not too long ago this number was in an acceptable range and I may have blown it off and moved on, looking for another reason for the drivability complaint.

On the verification test drive, intake air temperature behaved more normally — cooling instead of heating up. So this one caused the DTC and led to the cause of the drivability problem.

The only way you would really know is by collecting known good data. The data for me happened to be sitting in my driveway when I got home. Sometimes we get lucky but at other times we are waiting for information. In this case, using the technique that I recently learned to help me get to pinpointing the problem much quicker because I didn't immediately have a known good. Next time you’re on a road test for a low power complaint grab your intake air temperature and absolute load PID and some downstream oxygen sensor data, and you should very quickly be able to identify a restriction in the exhaust!

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