Check Engine Light On! What Do Those Codes Mean?


It was a chilly night in Montana, but crystal clear. I was standing next to my Honda, taking in the majesty of the great arc of the Milky Way. Using a cool app on my phone, I tracked the International Space Station and saw it fly above my head at 17,000 miles per hour. It’s times like these that make you feel pretty small in the universe. 

Especially when you’re stranded on an Interstate in the middle of nowhere with just a shred of cell phone service, waiting on a tow from AAA. 

It happens to everyone, but this was a breakdown I could have avoided. Why? Against the better judgement nagging me over and over in the back of my mind, I had been ignoring a check engine light (CEL) for weeks on my $600 Craigslist-special Honda CR-V


By themselves, CEL’s don’t do much for you, other than to light up an LED in your dash, and leave you scratching your head about what might be going wrong with your car. But, with a simple tool you can interpret that fault light, and take steps to remedy whatever problem you and your car may have. 

Let’s take a quick look at a handful of common fault codes and how to address them. But first, what is a CEL, exactly?

On Board Diagnostics

In the early 1990s, as cars began to rely more and more on computers for controlling all aspects of the vehicle, from engines, to transmissions, to brakes, auto manufacturers rolled out a universal electronic diagnostic tool that could talk to those computers. This tool plugged into a port located under the dash, and it could decode any problems the computers might have with the inner workings of the car. Early on board diagnostics were manufacturer-specific.

Known as OBD I, it was quickly succeeded by another version in 1996: OBD II. OBD II is universal, and is found in every car manufactured today. It is a powerful tool for helping mechanics diagnose all kinds of issues, across all kinds of brands. Professional shops have very expensive versions of OBD II scanners that produce highly detailed diagnostic read-outs.


What’s exciting in the twenty-first century for the home mechanic and the DIY’er is the availability of inexpensive hand-held OBD II scanners. So, when something goes awry with your car and that little orange light pops on, you can do some diagnostic work yourself in the comfort of your own garage. Many of these new OBD II scanners even connect to your smartphone via Bluetooth. In addition, you can also borrow a scanner for free from any of the major brick-and-mortar auto parts stores you might have in your town. Most scanners also have a function that allows you to cancel the CEL fault code, turning off that annoying light in the instrument cluster.

If you’re ready to up your diagnostic and data game for your ride, ScanGauge’s OBD II scanner and on-board display can display real-time data about your car as you drive along, in addition to interpreting fault codes from check engine lights.

ScanGuage OBD II Scanner

Some Common CEL Fault Codes

Now that you know what’s behind that mysterious CEL in the dash and how to crack open its code, here are a few common CELs that you might encounter.

P0455: Leak in the Evaporative emission control system (EVAP)

All modern vehicles have closed fuel systems – meaning that they are designed to prevent raw fuel vapors from escaping into the atmosphere. This is a good thing both from a safety standpoint, and an environmental one. As emissions laws have become more stringent over time, the sensitivity of these systems to leaks has increased, so P0455 is likely one of the most common CEL fault codes you’ll encounter.

It could indicate that you have a fuel leak somewhere in your fuel lines, or perhaps near your fuel injectors, or even a failing carbon canister – a component on many cars that chemically captures fuel vapors. 

Nine times out of ten? It means you forgot your fuel cap on top of the fuel pump, or didn’t tighten the cap down far enough. Remember to give it a few clicks when you tighten down.

P0300: Cylinder Misfires

A second CEL code that pops up quite often is P0300. This code covers a variety of possible conditions, and this is where those fancy professional OBD scanners mechanics use pay for themselves. An average consumer-level scanner only indicates a cylinder misfire, the really expensive ones can tell exactly what’s causing it.

For you and me, however, there are some usual suspects to track down when we get this fault code. An engine misfire usually indicates a problem with ignition first, and fuel supply second. A common reason for misfires in modern vehicles are failing ignition coils and spark plugs that are well past their prime. It can also suggest that your fuel injectors are not performing correctly, causing hesitation and stalling.

Ignition coils are a common cause of the P0300 fault code.

However, P0300 can also be a red flag for much more serious problems, including engine timing that’s off, a failing distributor, bad camshaft or crankshaft sensors, or, most troubling, poor compression and head gasket leaks. This is definitely one of those codes that may be worth having a mechanic check out – as many of these problems can sink your car for good if you don’t feel comfortable diagnosing them yourself.

P0420: Catalyst System Efficiency Below Threshold

And now, for my own problem on the side of a highway late at night in a Montana winter. 

Honda CR-V on mechanic's lift
The Rally CR-V on the lift at the only mechanic within 40 miles of where I broke down.

P0420 suggests that you have an exhaust issue – this sometimes can be with your oxygen sensors, but more often than not, it means your catalytic converter is failing. Catalytic converters are designed to filter out the worst of your exhaust gases before they reach your tailpipe, and after a while, they clog up and break apart internally. Backed up exhaust means your engine can’t breathe correctly, which means you lose power, and eventually come to a stop altogether.

My CEL was on when I bought the car, and when I scanned it the first time I decided to roll the dice that the problem might have been a bad oxygen sensor – a condition you can continue to drive on with relatively little problem. Unfortunately, that was not the case, and I sputtered to a stop about 25 miles from, well, anywhere.

The catalytic converter on my Honda lasted for 23 years and over 200,000 miles, which, in the grand scheme of things isn’t that bad, but I should have probably done something about it sooner. At least I saw some nice stars.

Broken catalytic converter
My 200,000 mile catalytic converter.

Exhaust work is a specialized skill, but should this happen to you, any local muffler shop will be able to install your new catalytic converter in an afternoon. Some cars have catalytic converters that can be unbolted from the exhaust, and a new one simply bolted in – which is well within the abilities of the average home mechanic. Always remember to change out those oxygen sensors at the same time, they’ll keep your car running smoothly into the future. 

What’s the most common CEL code you see on your car? Questions about other codes? Let us know in the comments!


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