Why is My Check Engine Light On?

You like to ignore your CELSo your Check Engine light is on and you’re wondering what it could mean. Let’s see if All Tune and Lube in Tempe can help out a little.

The first thing you need to do is find out why, right? Take your car down to a shop you trust to have the codes read. The parts stores will read the codes for you but they’re probably not going to be able to give you a feel for how serious the problem is, very serious or not so much of a problem. A regular auto repair shop is likely to have more experience and be able to provide a bit of guidance you’re not likely to get from a parts store. If anyone wants to charge you for reading codes on a car built after 1996 RUN AWAY! Cars built before that can be a little problematic.

Here are the five most common CEL codes:

  • Oxygen Sensor fault. Can be bank 1 or 2 (refers to the side of the engine if you have a V6, V8 or H4 or H6) or it can be sensor 1 or 2 if your car has sensors both upstream and downstream from the catalytic converter. Sensor 1, the upstream sensor, is also called a fuel air trim sensor. The computer that controls your engine gets information from that sensor that affects the way your car runs and your fuel economy. Sensor 2, the downstream sensor typically monitors the catalytic converter. Just about anyone with a little mechanical knowledge can replace an oxygen sensor and if you’re sure that’s the problem just go for it. That being said it’s not always that simple. Your shop has equipment that can trace the function of the oxygen sensor and make sure that’s the problem.
  • Gas cap loose, damaged or missing. OK, this one is a no brainer, right? Pretty much. Take a look the gas cap and you’ll see a rubber ring around it up at the top of the treaded part. If there are cracks replace it with an original equipment cap! The aftermarket caps just don’t seem to be up to snuff. From time to time we see this code come up when the cap hasn’t been tightened enough. Make sure you hear it click at least 3 times. If the light comes back on it could be that there’s an evaporative system leak that’s mimicking a fuel cap problem. Take it to a shop.
  • The third most common code we see is the dreaded P0420 Catalytic Converter Efficiency Below Operating Threshold. There are rare exceptions but you’ll probably need a new cat.
  • The next most common is a Mass Air Flow Sensor fault. Like the upstream oxygen sensor, this item provides information to your cars’ computer for engine management. MAF sensors don’t fail all that often but they do get dirty, especially if you don’t change your air filter often enough. These sensors are easily damaged and take a special cleaner. If that doesn’t take care of the problem take your vehicle to a shop (All Tune & Lube Tempe for example!) that has the equipment to test it. There’s not usually a lot of labor to the job but the part can get pricey.
  • The last are the misfire codes. All sorts of things can cause your engine to misfire. Spark plugs, ignition wires or COP boots, ignition coils and fuel injectors come immediately to mind. Don’t let anyone just do spark plugs and wires on your car until they’re sure of what the problem actually is.

There are all sorts of additional faults that can turn the Check Engine light on. If your car seems to run OK but the CEL is on, take it by your shop as soon as you have an opportunity. If your car starts running poorly or if the CEL is flashing get to the shop ASAP. I hope you’ll consider All Tune and Lube Tempe.