Troubleshooting MIL Issues: OBD II & “CAN” Scan Tool Diagnostics

Troubleshooting MIL Issues: OBD II & “CAN” Scan Tool Diagnostics

The Malfunction Indicator Lamp (MIL) is one of the most misunderstood warning lights on vehicles today. Most motorists have no idea what the light means or why it’s on. Some think the MIL light is an oil change reminder light. Others realize something must be amiss to cause the light to come on, but don’t know if they should keep driving or not. When others see the light, they just ignore it.

The MIL light is an emissions warning light. It’s not an engine maintenance reminder light, an overheating warning light, a low oil pressure warning light or a low charging voltage light. If the MIL is on, it means the onboard diagnostic system (OBD II) has detected a fault that may cause emissions to rise and has set a Diagnostic Trouble Code (DTC) that corresponds to the problem.

A MIL light doesn’t necessarily mean the vehicle is polluting. In fact, the vehicle may even pass a traditional tailpipe emissions test with the MIL on. But in states that do an OBD II plug-in check on 1996 and newer vehicles, a MIL light that is illuminated will cause the vehicle to fail the emissions inspection — regardless of what’s actually coming out of the tailpipe.

That’s because OBD II monitors a lot of other things besides the emission controls on the engine itself. It also monitors fuel and ignition as well as Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). It can tell if the engine is running rich or lean. OBD II also monitors evaporative emissions as well as converter efficiency. Any fault in any of the monitored systems that might cause emissions to increase 1.5 times the federal limit may set a DTC and turn on the MIL light.

One of the most valuable pieces of service information you can refer to when troubleshooting a DTC is the enabling criterion that causes a code to be set.

Unfortunately, this information is often difficult or impossible to find. Most vehicle manufacturer diagnostic charts for a given DTC will describe the step-by-step inspections and tests you need to perform to isolate the fault. But they may not always describe the operating conditions that are necessary to set the code in the first place. In some situations, not knowing the criteria that set the code may lead you down the wrong diagnostic path, causing you to replace the wrong part. A few days later the MIL is back on and your customer returns, unhappy that you failed to fix his problem the first time.

Every automaker determines its own enabling criteria and threshold points where they think a DTC should be set to turn on the MIL light. The set points for various codes are based on hundreds of hours of laboratory and dyno testing, and often vary depending on the vehicle application, engine and emissions calibration. The tougher the emission requirements, the more sensitive OBD II is to perceived emission faults, and the more it is to set a code and turn on the MIL light.

Between all the engineers, lawyers and bureaucrats who come up with this stuff, you’d think somebody would create a better means of alerting a motorist when his vehicle has a potential emissions problem. Instead of turning on a light that says “Check Engine” or “Service Engine Soon” or illuminating an icon of a motor, why not display a message like this:

“Your vehicle has a possible emissions fault. The fault is in the oxygen sensor (or whatever) circuit. It’s safe to continue driving your vehicle, but emissions, fuel economy and performance may be reduced. Also, your vehicle will not pass an emissions test as long as this fault is present. You should make a service appointment soon.” Such a message would be a much more informative approach than the current method of flashing an idiot light in the driver’s face. You would think some of the more technically advanced nameplates such as Mercedes or Lexus would take a more proactive approach toward onboard diagnostics, but we haven’t seen it yet. So until things change, somebody has to plug a code reader or scan tool into the vehicle’s diagnostic connector to read out the DTC(s) that turned on the MIL light.

Actually, that’s probably a good thing for most of our readers because it means motorists have to bring their vehicles to you to have their MIL light diagnosed. Their only other option is to spend $100 or more on a code reader or DIY scan tool that may or may not accurately read and display their vehicle’s diagnostic trouble codes. The limitation with many inexpensive code readers and even some basic-level DIY scan tools is that many of these tools read only generic or global OBD II “P0” codes. Many do not display OEM enhanced “P1” codes, or, if they do, the list of applications they cover may be limited to domestic makes or only a handful of Asian cars. What’s more, a code reader only displays and clears codes. It doesn’t display or capture data.

DIY scan tools can display some sensor data and many also show the status of the various OBD II system monitors, but the list of PIDs is typically quite limited compared to an OEM scan tool or a professional-grade aftermarket scan tool with up-to-date software. DIY scan tools do not allow bi-directional communication to run actuator and other system tests for liability reasons. So even if a motorist has a code reader or scan tool that can read codes, chances are he won’t have the technical know-how, experience or additional diagnostic tools to accurately troubleshoot his MIL problem.

Another thing many motorists don’t realize (as well as some technicians) is that a DTC is not a diagnosis. Most codes indicate a fault has been detected in a particular circuit or system (voltage too high, too low or out of range). Other codes may indicate the engine is operating out of its normal range (running too rich or too lean), or that the engine is misfiring. Some codes may also indicate a leak has been detected in an evaporate emissions control system (possibly a loose gas cap or a real leak), or that the operating efficiency of the catalytic converter has declined. But no code tells you which part needs to be replaced. Nor does any code tell you why the engine may be running rich or lean or misfiring.

A MIL light and a code only tells you OBD II has detected a fault. Consequently, additional diagnosis is almost always required before any parts can be replaced. If you think you’re smarter than the OBD II system and jump to conclusions, you run the risk of replacing the wrong part. Sooner or later the MIL light will be on again and your customer will be back.

Since model year 2003, a growing number of domestic and import vehicles have been built with a new onboard communications protocol called CAN (Controller Area Network). CAN is essentially an engineering standard for how computers and modules talk to one another via the serial data bus in a vehicle’s wiring system. It’s a high-speed standard designed for powertrain control modules, antilock brakes and stability control systems. The CAN protocol was created back in 1984 by Robert Bosch Corp. in anticipation of future advances in onboard electronics. The first production application was in 1992 on several Mercedes-Benz models, but CAN is now being used on more and more new vehicles. By 2008, all new vehicles sold in the U.S will be required to have a CAN-compliant onboard diagnostic system. Some recent import CAN applications include: 2003 Mazda 6 and Saab 9-3, 2004 Lexus LS430, Mazda 3 and RX-8, Toyota Prius and Volvo S40, and 2005 Audi A4 and A6, Isuzu Ascender, Lexus LS400 and GX470, Land Rover LR3, Mazda MPV and Tribute, Mercedes SLK350, Saab 9-7X, Toyota Avalon, Toyota 4Runner, Sequoia, Tacoma and Tundra, and Volvo S60, S80, V50, V70 and XC90.

CAN-equipped vehicles require a CAN-compliant scan tool for onboard diagnostics. So not only do you need up-to-date software in your scan tool to read codes and data, you also need the right internal hardware that can handle the faster communications protocols. Many older scan tools are not upgradeable to CAN, so that means buying a new CAN-compliant scan tool.

In CAN vehicles, information is shared over a high-speed serial data bus. The bus is the circuit that carries all the electronic chatter between the modules (nodes). On some high-end Mercedes models, the CAN bus may have more than 200 modules controlling various functions throughout the vehicle. That’s as many modules as there are on some commercial jet liners!

What’s more, Mercedes uses several different bus speeds on their vehicles. Depending on the application, there may be a high-speed 500 Kbps CAN-C bus for the powertrain, transmission and ABS modules, and a slower-speed 83 Kbps CAN-B bus for the body control functions. Up to model year 2002, all communication between the CAN-C and CAN-B bus went through the electronic ignition switch (EIS) module. After 2002, a new “gateway” module handles the inter-bus communications as well as onboard diagnostics via a CAN-D bus. In CAN-equipped cars, every module (node) that is attached to the data bus network is capable of sending and receiving signals. Each module (node) has its own unique address on the network. This allows the module to receive the inputs and data it needs to function, while ignoring information intended for other modules that share the network. When a module transmits information over the network, the information is coded so all the other modules recognize where it came from.

[See the September, 2006 issue of ImportCar, pages 34-38 for more details on CAN networks.]

When you’re troubleshooting a MIL light on a CAN-equipped car, keep in mind that faults in the CAN bus and network system can set codes that are the same as faults in sensors or other systems. Though CAN systems use fewer wires and fewer connectors (to save weight and cost), they also use more modules and more complicated operating strategies. Problems can occur if module connectors become corroded or loose, if wires become grounded, shorted or break, or system voltage is below specifications. Some modules may even forget their settings or locations if the battery is disconnected or goes dead. This may require a “relearn” or “reset” procedure with a scan tool if the battery is disconnected or power is lost for any reason.

Something else to keep in mind about CAN diagnostics is that modules send and receive “OK” signals to let the main control module know if they’re working or not. In theory, this makes diagnostics easier. On the other hand, it also means that one misbehaving module may generate enough noise to disrupt the entire network, causing a complete shutdown of the vehicle!

When a serial bus communication problem occurs, it will usually set a “U” code and turn on the MIL light. Depending on the fault, the vehicle may or may not start, or it may operate only in a “limp-in” mode with limited capabilities. Loss of communication between the engine controller and transmission controller may put the transmission into a limp-in mode where it will operate only in one or two gears.

Loss of communication codes may indicate a wiring problem on the bus, or a fault with a module. Isolating the fault may require unplugging modules one at a time until the fault is found. Just remember that all modules on a bus network need three things to function properly: power, ground and a serial data connection.

When diagnosing bus or module communication problems, you usually start by checking for voltage at the module, then the ground connection and finally the data line. If all three are good, but the module isn’t working, the module needs to be replaced.

To minimize the parasitic current drain on the battery when the vehicle is off, a “sleep” signal is sent to the modules on the network. Some modules may remain on for a short period of time after the ignition is switched off (air bag module, for example), while some may never go to sleep (anti-theft module and keyless entry receiver, for example), but most are put into a sleep mode to conserve battery power. If the sleep signal is never sent, or a module fails to recognize the sleep signal, it may remain active and pull power from the battery. Depending on the current draw, this may run down the battery if the vehicle sits for a period of time.

If a CAN-equipped car has a battery that keeps running down, therefore, you should measure battery current draw with the key off at various intervals after the engine has been turned off. If current draw exceeds specifications, there may be a faulty module that is never going to sleep and is drawing too much power.

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