Diagnosing Check Engine Light Issues

It's a submarine! It's a helicopter! It's an airplane!
No, it's an artist's rendition of your engine.

That pesky check engine light (CEL, otherwise known as the Malfunction Indicator Lamp/MIL in the business) is one of the characteristics of any modern car with computer control, and is enough to make most people pucker their rear o-ring and clutch their wallet tight when it remains lit after starting the car. Some people have been known to trade in their car for a new one based on this light.

Compared to cars older than a few decades ago, modern cars have much less emissions, start and run more reliably, and require less ongoing tuning and adjustment to keep running optimally.  The reason for this is computer control of almost every aspect of the engine.  The engine has an array of sensors and the computer continuously monitors readings such as air pressure, engine temperature, air temperature, throttle position, and oxygen content in the exhaust among many others, and in turn directs the fuel injectors to spray fuel into the engine and spark plugs to fire with precise timing depending on calculations from the sensors. Emissions systems are monitored to ensure they are working properly.

Most of the time this works pretty well, and modern ECMs (Electronic Control Modules) are extremely reliable.  Sensors, however, wear out, electrical wires can become frayed or corroded, mechanical parts can break and prevent proper operation of the engine, etc.  In these cases, the reading of a sensor can be outside the range that the ECM determines is normal.  For example, if an oxygen sensor is going bad the ECM is programmed to notice a sensor reading that doesn't change over time, indicating that the sensor has gone out to lunch and isn't coming back. This is much smarter than the ECM assuming the engine is constantly running too rich and leaning out the mixture until your burn your spark plugs up.

In this case, the computer will store a code, called a Diagnostic Trouble Code (DTC), indicating the exact issue and the MIL light will come on.  So at this point, what do you? Before you do anything, relax. This doesn't necessarily mean there is a major problem with the car.

The first step is to get the code.  This will narrow down the problem to one area of the car, such as a faulty transmission clutch switch or vacuum leak, or even just a loose gas cap.

There are a few different ways to get the stored fault codes.  What most people do is take it to the dealer where they ask you to turn around and touch your toes (fork over $100+) just to connect their scan tool.  Then they let you know how much more to fix the problem along with several hundred in other repairs they "recommend".  But you can start by reading the code yourself even if you end up taking it somewhere to get repaired. At least you'll have more information in hand when the time comes to hand over money.

Option 1: Use an OBDII scan tool

All modern cars implement OBDII (On-Board Diagnostics) which provides a standard way to obtain these DTC codes.  There are adapters available that plug into the OBDII port and the USB port of your laptop and allow you to read and/or clear any stored codes or view real time values from the computer such as engine RPM, oxygen sensor readings, vehicle speed, etc.

All you need to do is find the OBDII port on your car, plug in the cable, and run the software:

Viewing ECM parameters from an Acura TL. Arrow shows location of OBDII port
on this car.  Your may (probably will) vary.

Caveat: Consumer-grade scan tools many times will not read codes from subsystems such as faults with anti-lock brakes (ABS), traction control (TCS, VSA, and others), airbags (SRS), or other secondary systems. For those, keep reading.

Option 2: Use a scan tool from an auto parts store

Many stores, such as AutoZone, will allow you to borrow their scan tool in the parking lot and scan for fault codes.  If you're not comfortable using the tool, they will come out and do it for you, free of charge.

Their scan tool may support more systems than an entry level consumer model such as ABS, TCS, etc. depending on the make and model of the car.  It's a good idea to write down the codes and google them at home to see what other people have experienced with similar codes on your car before deciding to buy any expensive parts.

Option 3: Use the MIL to read the codes

Some cars allow you to jumper a pin in the OBDII socket to ground to activate a service check mode when you turn the ignition to ON.  For example, on Honda/Acura cars you can jumper pins 4 & 9 (connect them with a wire or paper clip... be sure to look up whether your car supports this or not, and be sure of the pin numbers) on the connector to read the codes.  Upon switching the ignition ON, one or more indicators on the dash will blink.

Your car may support checking codes via jumping pins 4 and 9
on the OBDII port.  Do a web search first to make sure before trying this.

This was the only way I could read the SRS code on my Acura TL recently, which turned out to be a problem with the OPDS in the passenger seat.  This system monitors the presence and position of the occupant in the passenger seat and will turn off the side airbag if it is not needed or the passenger is leaning too close to the side airbag.  This fault can be triggered by storing objects on the passenger seat and other electronics usage in the car. I didn't know this and routinely placed my laptop and other items on the seat while driving to work.  So, I cleared the code and so far it hasn't come back.   I'm not sure if that was the cause, but if the code never comes back that could have been it.

Incidentally, my brother just told me "Every fault that comes up on your car is some part that doesn't even exist on my car!"  So true! The more technology they pack into a car, the more parts there are to wear out and break over time.  If your car is older and a module goes bad that costs $1000 to replace, you may opt to just go without that feature (like ABS) if the whole car is only worth a few thousand.

Note: With this method, you will have to google for information on how to read the blinks.  Usually you count the number of blinks, a long pause separates digits, and sometimes a very long blink or many very quick blinks in a row means 10.  On my TL, the SRS code was 15-3.

No, no, no, no, no! My dash recently.  MIL, VSA, and SRS indicators all on. They
were all able to be cleared and appear to have just been temporary. 

Whichever method you use, now that you have the code, what do you do? Unless you're familiar enough with the car to know exactly what the code means, the best course of action at this point is to google the fault code along with your make & model of car.  Best case you come across a message board with information on how to track down the issue.

Another source of information would be either the Haynes or Chilton's shop manual for your car, or its factory service manual.  Either of these are invaluable in servicing your own car, as they will have detailed troubleshooting procedures and useful information for disassembly and reassembly of all parts of your car. Always have some good reliable information from the web or from a shop manual before you start a repair you're not familiar with.