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P0420 is now the leader of the pack

P0420 (Catalytic Converter Below Efficiency) has risen to the top of the pack in internet code searches. 9 factors that have nothing to do with performanve of the cat may trigger this code, they are.

1. Ignition timing, even if your timing is non-adjustable the sensors that set the timing could be in error.

2. Intake or exhaust leaks.

3. Engine Coolant temp. sensor

4. Contaminated oil.

5. Dripping fuel injectors

6. Inacurrate 02 sensors

7. Cyl misfire,a cylinder misfire can cause this code to be set and not set a misfire code.

8. Overall engine mechanical condition.

9. Missing cat. heat shield. A missing shield may prevent the cat. from reaching optimal temp.

Before you say “so what is your question” remember that an internet search will pull up this post also(along with all the others we make).

I wonder if the fact that “420” is the new term for marijuana has anything to do with its popularity in internet searches.

. . . And TPS, . . . And EGR System, Too, Just To Name A Couple More.

Oldschool, This Comment / Question You Submitted Is One Worth Further Discussion. As As A Public Service, Car Owners, DIYs & “Mechanics”, Should Be Made More Aware Of Misusing PO420.

Not knowing about timing belts / interference engines and the proper use and interpretation of PO420, I would say, probably cause the most unnecessary dollars wasted on car repairs / maintenance.

I wish I had a nickel for every car in the salvage yard that died an early death from a broken timing belt and I wish I had a nickel for every catalytic converter replaced because code PO420 was misconstrued as the grounds for cat replacement.

Back to PO420: This code is often seen as a cause and not a symptom. From experience, I have found that converters are pretty tough and will tolerate a code PO420 quite well while somebody tries to figure out what is going on upstream to set it off. As you indicated, there are many things and it doesn’t take much. Intermittent problems make finding the cause very difficult sometimes.

How many times do you suppose a code PO420 leads to converter replacement only to find that the PO420 code returns and other codes present themselves or reappear ? I have heard of cases where the “mechanic” actually returns the converter for warranty replacement and the code comes back, again.

Often other codes are present with PO420 and those need thorough investigation. Even when no other codes exist the P0420 still requires a thorough investigation of what’s going on upstream.


I agree with CSA. I have run across a dirty air filter a few times that was causing this code. Checking the engine upstream is very important unless you want to throw your money away.

My Vic had this disease…The dreaded P0420 affliction…Since I knew the rear oxygen sensor was the part throwing the code, and the car was running fine, I replaced the rear sensors and reset the CEL…it has stayed off…O2 sensors do NOT last forever…

It looks like CAR MD won’t cover it all or save anybody three times the purchase price in the first year. I bet that thing will get many people thrown out of repair shops.

The infomercial acts like it’s all so simple. Now, people with no training at all will be posing as experts. Oh, that’s been happening for centuries anyway.

The cat efficiency is a calculation against the up and down stream sensors. A bad upstream will not lean the system properly. A bad down will lie about the net O2 change across the cat. To me a o2 is about $60 to buy and an hour or five on a bad day to change. On a six cylinder you would have a chance to swap bank one and two sensors. A realtime code scan under load and coast against the o2 sensors could tell more about trimming the list, thats what I usually do.

So, how would you trouble shoot this code?

I know that it is a very common code for 2002 through 2006 Nissan Altima’s with the 2.5 4 cylinder engine. They have a tendency to have the catalyst fall apart in the pre-cat, which then gets sucked back into the combustion chamber during valve overlap, which then scores the cylinder walls, increasing the bore size, and causing excessive oil consumption.

By the time you get this code, its typically too late for the engine.

Easiest thing to do is to remove the pre-cat, and examine the catalyst for damage.

But, what a shop can do and what an individual can do are two different things.

On a shop level, having spare parts dedicated to testing would be the way I would go.
Swap out the O2 sensors first, clear the codes, then drive cycle the car to see if it comes back would be step one. That would at least eliminate the O2 sensors as being the culprit.

You also have to look at the way the car drives too, so it gets more and more complex.
If the owner hasn’t had the spark plugs changed in 150k miles, you would have to correct that first. But a shop might not know this is the case unless they get the owner to say so.
Or, unless they actually pull the spark plugs to perform a compression test right off the bat, and see the condition of them, but how many shops actually start with compression tests now a days, in actuality?

Certain codes are difficult to resolve cheaply.
This one has the potential to be really expensive.


This code can be ignored at zero cost if you are satisfied with the way the car is running and an emissions test is not an issue…If anything destructive is going on, there will be more than one code set in the computer…

For me the test with the hand held pyrometer gives the most usable data with the least effort (besides the rubber mallet test). Inlet and outlet on the cat. should differ by 100F (with the outlet being hotter). This is the kind of test that will tell all is well but if the temps are the same it does not mean conclusively that the cat is bad. If the temps do differ by the min. an “all is well” conclusion can be made

That is EXACTLY it. That’s the way search engines work and how they keep track of hits.

I think that all we can do is what we are doing. What I mean is all of our posts that are constructed correctly will get pulled up in an internet search, constructing posts that get pulled up in searches is how we can inform or make aware the most amount of people but it is in my eyes a rather crude method. The more detailed method is posting here and trying to warn people about these things. We are doing what we can but I am not saying we are doing all we can. Perhaps having a sort of agreement in our answers will get to the most people. I do know that having the marketing section of a business all working in agreement is the most effective marketing arrangement, our business is getting the word out.