When the “Check Engine” light comes on, many motorists don’t have a clue as to what to do next. They know the light means something is wrong, but is it a serious problem or a minor one? The only way to know for sure is to plug a scan tool or code reader into the vehicle’s diagnostic connector and see what comes out.

One of the first auto parts stores to offer a free plug-in diagnostic service for motorists who wanted to find out what was causing their “Check Engine” light to come on was AutoZone. The program is popular with motorists because it saves the cost of having to take the vehicle to a new car dealer or repair facility to have it diagnosed. Being able to tell customers why their “Check Engine” light is on opens the door for potential parts sales, and it helps the motorist to decide whether or not the problem is something they want to tackle themselves. At the very least, knowing why the “Check Engine” light is on allows the customer to be better informed should they decide to take their vehicle to a professional for repairs.

An Onboard Diagnostic II (OBD II) plug-in diagnosis is relatively simple to perform and requires only a code reader or basic scan tool. The tool is plugged into the diagnostic connector (which is usually located under the instrument panel near the steering column). To read the code(s) that are causing the “Check Engine” light to come on, you simply turn on the ignition, follow the prompts on the code reader or scan tool screen and read out the code(s).

The least expensive code readers only display a number, so you have to look up the code definition in a handbook to find out what it means. The more expensive tools give you the number and a brief definition. The trouble is, a code doesn’t always tell you what the underlying problem is, what might be causing it or what parts are needed to fix the problem. But, it is a starting place and gives you some direction as to what might be wrong with the vehicle so your customer can decide what he or she wants to do next.

Scan Tool Differences

Whether you are choosing a scan tool for diagnosing customer’s vehicles, or are recommending a scan tool to a DIYer or a professional, there are major differences between the types of scan tool products available.

There is a huge difference between a basic code reader and a full-fledged scan tool. A basic code reader that sells for less than $50 may be fine for reading “generic” diagnostic trouble codes, but it may not read “OEM” or “enhanced” codes that are vehicle specific.

There are two types of OBD II diagnostic trouble codes: generic “P0” codes and enhanced “P1” codes. All vehicles use the same basic definitions for the generic codes, but the enhanced codes are vehicle-specific and may vary from one model year to the next. So it will likely be out of date by the next model year and have to be updated or replaced. What’s more, some types of faults will set a P1 code but not set a P0 code. That’s why you need a tool that can read both types of codes, not just the generic ones.

The next step up in terms of capabilities are code readers that can also display the readiness status of the vehicle’s OBD II system monitors. These are self-checks that the OBD II system runs to make sure the vehicle is operating properly and is running clean. Some monitors run every time the engine is started and the vehicle is driven; however, others only run under certain operating conditions. One of these is the evaporative emissions system monitor (EVAP monitor) that checks for fuel vapor leaks. Many people call this the “loose gas cap monitor” because it may turn on the “Check Engine” light and set a P0440, P0442 or P0445 code if the gas cap is not fully tightened or is left off after refilling the tank. Another monitor that only runs occasionally is the catalyst monitor. This monitor checks the operating efficiency of the catalytic converter. If it senses a drop in efficiency, it will turn on the “Check Engine” light.

Why do you need to know readiness status? If all the monitors have run and no faults have been found (no codes set and no “Check Engine” light), it means the vehicle is in emissions compliance, is running clean and should pass either a tailpipe smog check or an OBD II plug-in emissions test.

Some people wonder why you can’t just clear the codes before taking an emissions test. Most code readers and scan tools can do just that. Pressing a button wipes the codes from the computer’s memory and turns off the “Check Engine” light. But as soon as the vehicle is driven, the code may reset and the “Check Engine” light may come back on depending on the nature of the problem. That’s where the readiness status comes in. A vehicle is not considered “ready” to take an OBD II plug-in emissions test unless all of the required monitors have run. The EVAP and catalyst monitor are the slowest ones to run, and may require driving the vehicle several days and at various speeds and loads before they will run their self-checks. So until that happens, the vehicle can’t be tested.

‘DIY’ Vs ‘PRO’ Scan Tools

A code reader can provide an inexpensive means of reading and clearing codes, and maybe even checking the status of the OBD II monitors, but that’s all. To read sensor data, history codes, pending codes, etc., you need a “real” scan tool, and the price jumps accordingly. An entry level scan tool with these features starts around $200 and goes up from there.

Most DIY scan tools do not have bidirectional communications capability. This is done for liability reasons. Most late-model vehicles have quite a few built-in self-checks that can be performed with a dealer or professional level scan tool, but running these tests requires some know-how, experience and caution. For many situations, the advanced capabilities are not necessary, but when it is needed, there’s no substitute for having a professional-grade scan tool. Pro tools typically start in the $1,500 to $2,000 range, and go up depending on the tool’s capabilities and features.

One change that is affecting the type of scan tool required to diagnose late-model vehicles is the introduction of Controller Area Network (CAN) computer systems. CAN engine control systems are much faster and smarter than previous generations of engine control systems. CAN is a high-speed data link that runs 50 times faster than the four existing OBD II communication protocols. The CAN protocol allows multiple control modules to share information, and requires special hardware and software for diagnostics. Consequently, you need a scan tool that is CAN-compliant for some 2003 vehicles, and many 2004-and-newer cars and trucks. Most older scan tools that were manufactured before CAN came along do not have the hardware capabilities to read the faster data and cannot be upgraded.

Understanding Codes

Regardless of what type of code reader or scan tool is used to read codes, an important point to remember is that a code by itself is not a diagnosis. It is a starting point for further testing and diagnosis. The code may tell you the nature of the problem or the circuit that is acting up, but it doesn’t tell you what’s causing it. This is why you can’t necessarily sell a part based on the code that was pulled from a vehicle.

The “enable criteria” that sets a code will vary from one vehicle to another, so there are no rules of thumb to guide your diagnosis. You usually need the OEM service information, diagnostic charts, and test specifications to isolate the fault. You also need additional test equipment such as a digital multimeter (possibly a graphing multimeter), digital storage oscilloscope and/or five-gas emissions analyzer to accurately diagnose some driveability and emissions problems — plus a thorough understanding of engine control systems, sensors and onboard diagnostics. In other words, the motorist really should take his or her vehicle to a professional shop.

Some higher-end scan tools not only display codes and data, but can also graph sensor waveforms. The ability to graph data makes it easier to see what is actually going on and to compare data.

Scan Tool Trends

In recent years, more and more new diagnostic tools have been introduced from companies such Actron (OTC), Auterra, Autologic, AutoXray (OTC), Baum Tools, Delphi and others. Their products range from entry level code readers to professional grade scan tools that combine scan tool, graphing and emissions functions all in one. Competition is driving down prices and forcing scan tool suppliers to include more features at little or no extra cost. This includes larger displays, color graphics, broader vehicle make and model applications (though European coverage is still limited in most tools to generic OBD II compliant only) and more parameter identification data. This is good news because it means scan tool users get more for their money.

Menus have also improved, making the tools much easier to use. Larger screens and color graphics also make it easier to see the data even in bright sunlight. Good visibility reduces the risk of misreading information and is a plus for aging boomers whose eyes may not be as sharp as they were 20 years ago.

Another trend we’re seeing is the narrowing gap between OEM dealer scan tools and aftermarket scan tools. A gap still exists and some aftermarket versions of the OEM scan tool have been “decontented”, but other aftermarket scan tools now offer most if not all of the diagnostic capabilities of the factory scan tool.

Most professional technicians say the factory OEM scan tools are the best, but can’t afford to own a different scan tool for each and every make of vehicle they work on. So they may have one or two OEM scan tools for the more common makes they service, and an aftermarket “all makes and models” scan tool for the rest.

Finally, there is the issue of tool obsolescence. Most scan tools are upgradeable with a plug-in cartridge, memory card or with software downloads. The tool manufacturer may offer free upgrades for a certain period of time, or they may charge an arm and a leg for annual updates. These are important things that need to be considered before a tool is purchased.


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