False Check Engine Light? Worth fixing?



Bought a car from a dealer, and about 2 weeks later check engine light came on. Checked it out and the code was p0420 the dealer took it to a mechanic that ended up replacing 3 o2 sensors and pipe connected to the intake that had a hole in it. Code came back again later and so a different mechanic performed a cat test and the readings were fine, he cleaned out the fuel system, added additives, and added premium fuel to filter out. The mechanic told me if it still comes off it has to be some kind of false reading. My question is, all the parts and systems run fine, is it worth the time and money to get an electrician to go in and figure out the issue?


What year and how many miles on this Fusion?



2006 and 114k miles on it.


I will compare it to an engine fire warning light in an aircraft staying on when there is no fire. What happens if there is a fire? In your case the check engine light being on and ignored will not kill you but might kill your car.


I mean I’d obviously check up on it every month or so to make sure no new codes form.


The code indicates that the catalyst efficiency is blow threshold.

All these so-called mechanics did was do everything but replace the cat. Which is the actual problem.



Like I said one mechanic did test the cat and said the test ran perfectly fine, even cleaned it and noted it wasn’t even that dirty. They could just be trying to dodge replacing it I guess.


That so-called mechanic also added premium fuel to clean out the cat.

Premium fuel does absolutely nothing in as far as cleaning out a cat.

You need a new mechanic.



I’m guessing the premium fuel was to clean the fuel system, but I agree both were shitty mechanics. Unfortunately, they are the only two the dealer covers under warranty.


Regular fuel has the same amount of detergents as premium fuel.

So that’s also BOOOOOOGUS!



That’s the key. The only two that the dealer uses. Dealer doesn’t want to pay to fix the car, tells their “mechanics” that they aren’t going to pay for it, so then the “mechanics” tell you nothing is wrong until the warranty expires.

I be the mechanics would sing a different tune if you were footing the bill on this repair.

(and please watch your language. This is a family type forum.)


You may have found the reason someone traded this car in, unfortunately.

Sounds to me like you need a new catalytic converter. Don’t go the dealer for this; their prices for CC replacement are sky high. Head to a local muffler shop and get a quote. I suspect you’ll be pleasantly surprised. Good luck.


You’ve been Bulls*****d.
I’m curious. I’ve worked with rhodium and platinum-palladium coated ceramic spheres while making laboratory hydrocarbon scrubbers, the same exact technology used in catalytic converters. How does he claim to have cleaned the cat converter? How does he claim to know how clean it is inside the canister where the real chemical magic is performed?

In short, you need a new cat converter… and you’re being “snowed”, as we say in New England… wicked-bad snowed as they say in Boston.


My check engine light has been on for over 170,000 miles. I checked it a long time ago and realized it was a non issue. I assume it is still a non issue because the car is still cruising along at 260K miles today and I drive it over 120 miles on my daily commute. It is nice to not stress out about more stuff than I already have to stress out about.

I have grown to disregard them unless there is a real issue with the car. I am fond on one of my old motorcycles choppers that not only has no idiot lights, it also has no gauges or signals. No oil pressure, no fuel level, no tachometer, no speedometer, no sensors to fail. Just hop on an ride worry free as long as there is oil in engine, gas in the tank, and air in the tires.


A personal choice that I support your right to make, but not one I’d recommend to others. :slightly_smiling_face:


My guess, the cat needs to be replaced. The o2 sensor involvement is b/c the ECM checks the cat operation by comparing the readings of pre cat o2 sensor to a post cat o2 sensor. If the cat isn’t doing anything, those two readings will be the same. If the cat is doing its job, they’ll differ in a known way. There isn’t a simple way to test the cat by itself, but well trained mechanics can use a factory scan tool or a lab o’scope and independently measure and compare the waveforms from the two o2 sensors as the engine is running to determine if the problem is the cat or the o2 sensors. If there was any doubt in how to interpret the waveform comparison, they’d do the same test on a fusion that had a known good cat for comparison.

I wouldn’t recommend ignoring the CEL and leaving it on, b/c if later something else serious occurs with the engine or transmission, a problem that might damage the powertrain quickly, you may not notice it happening.


I completely agree with that advice

Many times people have said the check engine light’s been on for years, it came on for such and such reason, and they never bothered to fix it, because it was something “non-critical” such as oxygen sensor or evap. And in the meantime, other things have gone wrong, now they have 5 fault codes, some unrelated to the original unresolved problem


That is what I refer to as cascading fault codes, and nothing good can possibly come from having multiple, unrelated codes being added to the system’s memory as time goes on.


If they live in a state with no inspection and don’t want to spend money fixing problems that don’t bother them, it’s ok with me. That car with 5 fault codes could get totaled in an accident at any time and look how much money the would have saved. Cars have limited lifespans, one of the worst feeling is having spent a lot of money on a car just months before it goes to the junkyard.
I remember my son putting 2 new fenders on a great running, stick shift, 318, Plymouth Valiant and having it painter only a month before the front tires started squealing. He had it aligned (badly, but that is another story) but the squealing continued. I took it to a good front end shop where the mechanic discovered that what looked like a good subframe was only undercoating over a rusted out shell.


I agree with you, this is not the way I would care for my car. I also don’t support people who make that choice because a CEL often means an emissions problem which affects all of us who breathe. I agree with the other commenters who point out that ignoring the CEL keeps you from knowing if there’s an additional code later. My one quibble with CELs, vehicles over 100K will show a code indicating an emissions component has failed but there’s no diagnostic to figure out which one so that leaves us changing parts till we accidentally or luckily hit on the right one. When that happened to me my saving grace was a dealer parts manager who knew from experience which of the 4 components were most likely the problem for that model.