Do you need a Diagnostic Scan Tool?

You are cruising along the open road. Traffic is light and you are making good time toward your destination. The stereo is blasting your favorite tune and the kids are behaving in the back seat. Then without warning that ugly amber colored warning light comes on: CHECK ENGINE. Or some variation such as Service engine soon.

Is it time to panic? If you have seen this light before you may be familiar with what to do next. If its the first time you have seen this light come on while driving you might become a little worried. The light may or may not accompany a change in engine performance. In other words, the car may continue to run fine as though nothing is wrong. In some cases engine power may be reduced while the light is on. In either case what do you do?

Most all other warning lights in the car such as BRAKE, TEMP, or OIL are red when they illuminate. This of course means to stop the vehicle immediately. The CHECK ENGINE light is amber in color meaning caution. There is no immediate danger to the vehicle however it needs to be serviced as soon as possible. But what exactly does this light mean?

All modern cars use an onboard computer to control engine functions such as fuel delivery and spark. Newer cars have several onboard computers however the one responsible for controlling the engine is known as the Powertrain Control Module, or simply known as the PCM. The PCM receives input signals from various electronic sensors on the engine and the exhaust system. These sensors measure such things as the temperature of the engine, the angle of the throttle (How much you have your foot buried into the gas pedal), the amount of fuel in the exhaust, and the speed of the car. There are as many as 15 or more input sensors on modern cars and all these sensors report their reading to the PCM. In return the PCM uses this information to determine output controls such as how much fuel the engine needs and when to fire the spark plugs. This allows the engine to perform with the best possible performance, fuel economy and lower emissions. In a future blog I plan to cover this a little more in depth.

The PCM has a strategy built into it. In other words the computer compares the signals it receives from all the sensors to the strategy programmed into it. If one or more of the signals sent to the PCM doesn’t jive with the strategy a code is logged for that failure and the check engine light is turned on.

Using a Diagnostic Scan Tool a mechanic is able to connect to the PCM and read what was logged in the form of a Diagnostic Trouble Code (DTC). This code identifies the sensor and circuit that is sending the funky reading that doesn’t match the strategy programmed into the PCM.

Can you as the car owner do the same thing? Most certainly. Diagnostic Scan Tools are available at almost all auto parts stores and have come down in price. A nice hand held scan tool capable of retrieving and clearing DTCs can be found for as little as $50 online and at some parts stores. This is compared to several hundred dollars just a few years ago. Some auto parts stores may even loan you a scan tool or offer to retrieve the DTC for you.

But what information does this provide you? There hundreds of Diagnostic Trouble Codes capable of being stored in most PCMs to cover just about any failure logged into its memory. Some of these are fairly straightforward, others a little more complicated. If you plug a scan tool into your PCM diagnostic connector the code or code you retrieve will only tell you the sensor or circuit responsible for turning on the CHECK ENGINE light. As an example you might retrieve information on the display such as: P0320 Ignition/Distributor Engine Speed Input Circuit. 

Where do you go from there? Once a trouble code is retrieved there is additional diagnostic information to trace down the cause of that failure. Is it a sensor? Is it the wiring to a sensor? Or something else?

There are many publications online as well as in books to help diagnose DTCs retrieved from a Powertrain Control Module. But this is only the beginning. The Diagnostic Scan Tools owned by professional repair shops are capable of retrieving the vast amount of information stored in the PCM to aid in diagnosis. These often show live data while the engine is running as well as the data that was stored when the fault occurred. The scan tools capable of providing this information start at about $2,000. A diagnostic flow chart and a scan tool capable of reading data are a must when diagnosing the cause of a CHECK ENGINE LIGHT.

But should you own a hand held scan tool capable of only retrieving Diagnostic Trouble Codes? In my opinion it is not a high priority as it is only good for information. Your money may be better spent on other tools. However it is handy to have to retrieve the Diagnostic Trouble Code before taking it to the shop. This may help you understand what the repair shop is dealing with. This may also give the shop a “heads up” that you are a car owner with some kind of knowledge of their car. If the Diagnostic Scan Tool has a feature to clear the code and turn off the light, do not and I repeat DO NOT clear the code after you retrieve it. This may also clear any data that was stored when the failure occurred. Doing so will make your mechanic dislike you very much and this may be reflected on your repair bill.