Check engine light been on. I checked gas cap the other day and it stayed on. I will put premium fuel in next time i fill up. It's been on for about a week. Any ideas to cause or solution?

  • Well, it's a code for poor catalytic converter efficiency, so the first thing to check is if it's still doing it's job. Jun 15, 2016 at 15:43
  • how many miles on the car? it may still be up for the federal emissions warranty. My friend just went through this on his 2011.
    – Ben
    Jun 15, 2016 at 16:38

3 Answers 3


P0420 is probably the most common code seen. It refers to "Catalytic Converter Efficiency"

Basically, it Engine Control Module (ECU) monitors the chemical constituents of the exhaust, both before and after the catalytic converter. The Catalyst is really a small furnace that reburns the exhaust and adds more oxygen where necessary, to eliminate harmful emissions and convert them into [hopefully] mere carbon dioxide and water.

If the ECU doesn't see the proper changes in the exhaust, eventually it will set the Check Engine Light with a code of P0420.

While being one of the most common codes, it can also be one of the most inaccurate. Perhaps your catalytic converter is failing, or "poisoned" from chemicals that were not supposed to be in the exhaust. More likely is the the rear oxygen sensor is failing or contaminated, giving false reports to the ECU about the catalyst health. However, unless your 2011 Equinox has a great deal of mileage, I doubt either of these explantions. The code can be cleared for free by a local auto parts store, or you can purchase or borrow a scan tool very inexpensively that will allow you to clear this code.

You can also complete several so-called "drive cycles" GM Example in an attempt to clear the code, but this can be a considerable time and effort sometimes.

You might also purchase one of several "magic juice" solutions at the auto parts store, which can clean and improve, to a limited extent, a tired catalytic converter. But a long steady highway drive should do the same.

If the Check Engine Light goes away but later returns, further diagnostics will be necssary due to the ubiquitous nature of this code.

Other posters are correct: if the catalyst needs to be replaced, it may be covered under the 8 year / 80,000 mile extended emissions warranty.

  • I doesn't matter if you clear the code, the problem will not go away, and, depending on the jurisdiction, the car will not pass emissions test if the monitors are not ready. When the code is cleared the car will have to do again the diagnosis of the emissions system which involve driving the car at certain speeds for a certain time (depends on the model/engine). Honestly, this is a great source of confusion. The check engine light was not supposed to turn off while the diagnosis is not ready. Jun 15, 2016 at 18:42
  • @Gabriel Diego Not every P0420 is a perfectly accurate, irrefutable analysis of catalytic converter health. Prolonged improper Air/Fuel Ratios can temporarily deplete the Oxygen Storage Capacity in a catalyst, the odd occasional glitch in a drive-cycle monitor, and a host of other things. I was not suggesting circumventing an emissions jurisdiction, nor was that what the OP asked. I would not "throw a cat" at a customer car, without first duplicating the CEL, and investigation other running conditions as you succinctly mentioned in your answer.
    – SteveRacer
    Jun 15, 2016 at 21:15
  • I didn't mean to say that you are suggesting that. I just mean that it is frustrating that the check engine light can be tampered so easily to foul people who don't have access to an OBD2 scanner that the car is working perfectly while the problem still is latent and may come back sooner or later. I know many people that told me: "I bought car and the CEL came on after only 15 miles I drove it. I must be so unlucky". I agree that sometimes the CEL is spurious and may go away by itself (or after clearing it). Jun 15, 2016 at 21:24

There is actually a letter that was sent out (I also have a 2011 Chevy Equinox with this issue and just had my cat. conv. replaced because of it) saying there may be thermal damage or melting on it and if you have this issue to take it to your dealer to get it repaired. Did you get that letter?


It can be many things

I mean, way many things, as SteveRacer has already cited. The most common (but not unique) is a failing catalytic converter. Please note that often a bad catalytic converter usually is a symptom of a problem, not a cause. If you replace it without solving the source problem, it will fail again in very short time. If this is the case and this came on recently you can try to use a fluid to clean the emissions system to solve this problem, at least for some time to buy you time (or to do smog, if it is required in your jurisdiction). I used Seafoam once and it solved for a few dozen miles.

Using premium fuel will do nothing

Only use the specified fuel grade for your car. Higher octane level in the gas is to prevent it to flashing prematurely in a high compression engine. If your car uses 87 gas and you put 91, you will just be wasting money.

Check if the warranty is still valid for the catalytic converter

In the United States (should be your case, right?), the catalytic converter is warranted by law for 8 years or 80000 miles (whichever first occurs) and if your car is within this warranty it is illegal to remove the catalytic converter yourself. You have to bring to the dealer and they should do it for free. I think that in some states (like California) the warranty is even longer.

  • No idea what you mean "the gas may not burn completely due to lower compression" Have any factual reference information on this? While I agree you should use a RON rating consistent with the owner's manual, I've never seen admonition to not use Premium gasoline. Poison a cat with oil or coolant, overheat and melt the ceramics by running lean, but I sincerely doubt octane or other knock additives (MTBE, ethanol) damage catalysts. I also do not agree with your assertion that compression has anything to do with complete combustion, unless there is a pre-detonation issue.
    – SteveRacer
    Jun 15, 2016 at 21:06
  • Higher octane levels mean higher resilience to spontaneous combustion. That means that to burn a high octane fuel you need an engine that operates at higher compression or the combustion may (or might) not be complete. This does not kills the catalytic converter instantly (or alone) and does not even happens to all cars, so the biggest risk involved actually is to waste money on a more expensive fuel with no return. Jun 15, 2016 at 21:10
  • Sorry, I simply do not agree. Need a good spark, that is all. Higher octane may be more friendly in a high-combustion application, but they do not mandate higher compression. the retardants resist PRE-detonation, knock, ping -- combustion initiated WITHOUT spark before TDC, that ruins hp and can cause damage. While I agree higher octane or RON fuel has less energy content (BTUs), that doesn't mean it can only be ignited by spark under higher compression. When operating Closed Loop ECU is adjusting air/fuel ratios many times a second. Only missfires P030x would leave HC in the exhaust.
    – SteveRacer
    Jun 15, 2016 at 21:29
  • I had read an article that this is an hypothetical situation. I will remove it from the answer to avoid misleading. Jun 15, 2016 at 22:08
  • Thanks, excellent edit. +1 If you can find the article, I'd be interested to read it. And I too agree that unless the ECU actively monitors knock, and can advance timing when using higher octane gasoline, there is no advantage and a huge drain on the wallet. I use 91 octane in my BMW 2002tii F production race car, because it is cheap, doesn't knock, and has MORE energy content (BTUs/gallon) than 110 octane racing fuel.
    – SteveRacer
    Jun 15, 2016 at 22:15

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