Since 1996, all automobiles and light duty trucks being sold in the US must come equipped with On Board Diagnostics Generation II Emissions Control Systems, hence the acronym OBD-II. OBD-II constantly monitors the performance of your Emissions Control System and will turn on your Check Engine Light whenever a malfunction occurs that causes the vehicle produced emissions to be greater than or equal to 150% of the Federal EPA mandated limits.
Steps
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Purchase an OBD-II scan tool from Amazon or a reputable auto-parts store. A capable “for home use” OBD-II scanner can be purchased for around $100. (If your car/light truck is older than 1996 you will need to purchase an OBD-I scanner which are more vehicle specific and do not use the universal OBD-II coding system. This article concentrates on the OBD-II system.)
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Locate the Diagnostic Link Connector or DLC in your vehicle. This is a somewhat triangular shaped 16-pin connector that is commonly located underneath the left hand side of the dash near the steering column. If you have trouble locating the DLC, use a search engine and enter something like “1998 Toyota Camry DLC.” There are lots of pictured DLC locations on the web.
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Insert the the scan tool connector into your 16-pin DLC and turn your ignition on, but do not start your engine. You will see the scanner begin to “talk” to the on board computers in your vehicle. Messages like “searching for protocol” and “establishing data transmission link” may appear on the scanner’s screen.
- If the screen stays blank and does not light up, jiggle the connector to achieve a better contact between the scanner and DLC connector pins.
- If you still aren’t having luck, be sure that your cigar lighter works. This is because the OBD-II system uses the cigar lighter circuit to provide voltage to the DLC. If the cigar lighter does not work, locate and check the appropriate fuse.
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When the scanner finishes booting up, look for a menu. Select codes and go to the codes menu. Depending on your scanner and year of the vehicle you may be presented with a few systems such as Engine/Powertrain, Transmission, Airbag, Brakes etc. For this demonstration, choose the engine and/or powertrain menu. There you will be offered 2 or more types of codes. The most common are active codes and pending codes.
- Active codes are live codes or malfunctions, that are keeping your Check Engine Light on, or turning it on and off. Just because your Check Engine Light is off doesn’t mean the code or malfunction disappeared, it just means that the code setting conditions haven’t occurred for two or more operations of the vehicle.
- Pending codes mean that the OBD-II monitoring system has failed the operation of an emission control system at least once and if it fails again the Check Engine Light will be turned on and the malfunction becomes a present condition or “acute” if put in medical terms.
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Break down an example of an OBD-II code to understand what the letters and numbers mean. P0301 indicates a misfire condition on cylinder #1. The P indicates it’s a powertrain code, the 0 indicates that it is a generic or universal code. If there was a 1 in its place, this would mean that the code is a manufacturer specific code and occurs say, on Hondas. The 3 means the area or subsystem is an Ignition System code. If it was a P0341, it would indicate that the Ignition System Camshaft Sensor circuit was defective.
- A code does not tell you what component is defective, it only points to or indicates that a component, its circuit, or its wiring/vacuum control are malfunctioning. The code may be the symptom of a malfunction caused by a completely different system. The 01 indicates it’s a cylinder specific problem, in that there is a misfire condition in the number 1 cylinder. It could mean that the spark plug, plug wire or dedicated ignition coil are worn out or that there is a vacuum leak near the cylinder.
- DO NOT think that the code tells you which part to replace. The friendly but usually untrained auto parts clerk will be happy to suggest many parts to try, but this can get very expensive and even confuse the issue.
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Hone your craft. The proper diagnosis of OBD-II codes takes years of training and practice. For example, you may have several codes that are being caused by a system that has not even set a code! A weak battery or worn out alternator can set 5 or more codes in systems that are perfectly normal. Take your car to an ASE Certified Master Technician with the L1 Advanced Engine Performance Diagnostic certification, or you could end up wasting a lot of time and money.
You’ll Need
- An OBD-II codes scanner tool.