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Check Engine Light basics

Check Engine Light Basics
People have different reactions to the Check Engine Light (CEL) when it suddenly illuminates for what seems to be no reason. These range from, if I ignore it, it will go away, to stopping dead in their tracks thinking the car is going to explode. These days its easy to have the codes read for free at your local auto parts store or you can purchase an inexpensive code reader to the job. Keep in mind the codes only give direction in diagnosing and don’t always indicate what part has failed. Lets explore some of the basics of why your Check Engine Light may come on and what to do about it.
 Your On Board Diagnostic (OBD) system consists of many components that monitor and control engine and transmission functions. When something goes wrong that will cause the car to emit excessive emissions the light will come on. There is probably thousands of possible causes that will do this, but we will cover some of the most common problems here. It is also important to understand that there are continuously monitored systems and other systems that are only tested after a certain drive cycle is completed.  This is why if you have the light cleared without fixing the problem it will stay out for a short time, once the drive cycle is complete for the failed system the light will return. 
1) Evaporative or EVAP codes P0440 - P0459, These codes are telling you your fuel system may be leaking raw fuel vapor into the atmosphere or the system itself is failing the ability to self test. The problem can be as simple as a loose or defective fuel cap or a more complex problem with the systems switching components or even a rusty filler neck. Your car can still be driven safely with EVAP codes stored, but should be repair to meet federal emission laws. 
2) Misfire codes. P0300 – P0312. This is a continuously monitored system. If you have a misfire you will see the light blinking as the misfire is happening. P0300 is a random misfire and P0301-12 indicate what cylinder is the offending one. This condition will cause your car to run poorly and possibly damage other components like the catalytic converter. You should have this check immediately to prevent further damage.
3) P0420- P0439. These are your catalyst efficiency codes. The cat has to be at peak efficiency to do it’s job. As the cat ages or is damaged from a misfiring engine and the efficiency drops to 92% or less the CEL will come on. This can be a very costly code to fix as it almost always results in cat replacement, but sometimes something as simple as an exhaust leak or the ECU needing a reprogram will cause the codes. It is very important do have these codes diagnosed properly before any parts are replaced due to the expense. Your car can be driven safely with these codes until repairs are made unless the cat is completely melted down causing an exhaust restriction.
4) Oxygen sensor codes P0130- P0167. There are many O2 sensor codes because there are several sensors on your car and they are responsible for reporting fuel mixture to the ECU as well as how efficient the cat is , a very important job. Depending on the engine configuration you will have two or four O2 sensors.  There are upstream sensors mounted in front of the cat and downstream sensors mounted after the cat. The sensors themselves have heaters to aid warm up. O2 sensor heater failure is very common. Once again the code must be diagnosed to get to the true cause of the problem. Your car can be driven with O2 heater codes, but if the engine is running poorly with a large fuel economy drop due to sensor failure it should be driven directly to the repair shop to prevent further damage.
Please keep in mind this is just general information and not manufacturer specific. This is meant to give a basic idea of why your CEL may have come on. We always recommend having your OBD system properly diagnosed before any parts are replaced.                          


2 Comments to Check Engine Light basics:

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