On many vehicles the PCM also controls the transmission. But on some vehicles, a separate transmission control module (TCM) is used to oversee gear changes and the torque converter. But even if there’s a separate module for the transmission, the PCM and TCM talk to each other and share data so each knows what the other is doing.

On many newer vehicles, the PCM also regulates charging system voltage; cycles the cooling fan on and off; interacts with the antilock brake system (ABS) module to reduce power if the vehicle has traction control; and may even interact with the automatic temperature control (ATC) module to operate the cycling of the air conditioning compressor clutch. The PCM may also be assigned vehicle security tasks.

One of the PCM’s most important jobs is to make sure all the engine’s sensors are working properly and that the engine isn’t polluting. Since the earliest days of the onboard computer, a certain amount of self-diagnostic capability has always been required to detect problems that might upset the smooth operation of the system. If a fault is detected, the PCM will set a trouble code.

On older vehicles, the diagnostics for the engine control system were relatively crude. If a sensor circuit went open (no signal) or shorted, the gross failure would set a trouble code and turn on the check engine light. But many conditions that didn’t cause a total failure could also upset engine performance and drivability. What’s more, the earlier systems had no way of monitoring many conditions that could increase pollution. So the Environmental Protection Agency (EPA) required every city and state that didn’t meet Federal clean air standards to institute some type of vehicle emissions inspection program.

Emissions testing has certainly helped boost the sales of aftermarket PCMs, sensors and emission control parts. But more importantly, it has made a significant improvement in the air quality of most large metropolitan areas. Even so, many motorists will only seek repairs if forced to do so because their vehicle failed an emissions test. Many put off repairs until their vehicle is barely drivable or dies and leaves them stranded.

With computerized engine control systems, it doesn’t take much of a sensor input problem to adversely affect driveability and emissions. A sluggish O2 sensor, a defective coolant sensor that always stays cold, a throttle position sensor that has a dead spot, an airflow sensor that isn’t reading accurately, etc., can all hurt performance, fuel economy and emissions.

In an attempt to ratchet up the self-diagnostic capability of PCMs, the California Air Resources Board developed a “next generation” onboard diagnostic system called OBD II. “OBD” is an acronym for “On Board Diagnostics.” The “2” stands for “second-generation system.” OBD II first appeared in 1994, and it has been required on all cars and light trucks since 1996.

Unlike earlier onboard diagnostic systems that set a diagnostic trouble code only when a sensor failed or read out of range, OBD II monitors most engine functions while the vehicle is being driven. It is designed to detect almost any problem that can cause emissions to exceed the federal limit by 1.5 times.

OBD II is extremely sensitive. Some say it is overly sensitive because the vehicle manufacturers have been overly cautious in setting trigger points below the 1.5 threshold to reduce the risk of expensive emission recalls. As a result, some vehicles may not actually have an emissions problem when the Check Engine light is on. Nevertheless, the problem should always be investigated to determine the cause.

The Check Engine Light (Malfunction Indicator Lamp or MIL) is supposed to alert the driver when an emissions or sensor problem occurs. Depending on how the system is configured and the nature of the problem, the lamp may come on and go off, remain on continuously or flash – all of which can be very confusing because you have no way of knowing what the light means. Is it a serious problem or not? If the engine seems to be running okay, should you ignore the light or not?

To address this issue, AutoZone recently announced a nationwide “Check Engine Light Program” for its stores. When a motorist has a Check Engine light on, he can take his vehicle to an AutoZone store for a free diagnosis. A store employee plugs a code reader or basic scan tool into the vehicle’s diagnostic connector and reads out the code. In theory, this provides a diagnosis so the appropriate part(s) can be replaced.

Unfortunately, it’s not as simple as it sounds. A trouble code is only a starting place. It’s not the final diagnosis. Somebody still has to check out the various components in the affected circuit to determine exactly what is causing the problem. This often requires following a lengthy diagnostic chart to isolate the fault. Jumping to conclusions often results in a faulty diagnosis.

For example, let’s say a vehicle has an OBD II code for the oxygen sensor circuit (code P0130). The code might indicate a bad sensor, or it might indicate a loose connector or wiring problem.

Harder to diagnose are misfire codes. OBD II can detect misfires in individual cylinders as well as random misfires. If it generates a misfire code for a single cylinder (say P0301 for the #1 cylinder), it only tells you the cylinder is misfiring – not why. The underlying cause could be a bad spark plug, a bad plug wire, a weak coil on a distributorless ignition system (DIS) or coil-on-plug (COP) ignition system, a dirty or dead fuel injector or a compression problem (bad valve, leaky head gasket, rounded cam lobe, etc.). As you can see, there are multiple possibilities, so it takes some diagnostic expertise to isolate the fault before any parts can be replaced.

random misfire code (P0300) is even harder to diagnose because there can be numerous causes. A random misfire usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty injectors, low fuel pressure, a weak ignition coil, bad plug wires or compression problems.

Something else to keep in mind about OBD II fault codes is that some codes are false codes. GM has had problems with certain 3.8L engines setting P1406 codes, which indicates a fault in the EGR valve. Replacing the EGR valve doesn’t fix the problem because the OBD II system is overly sensitive to how quickly the EGR valve opens when it is commanded to do so by the PCM. The cure here is not to replace the EGR valve but to “flash reprogram” the computer so it is less sensitive to this condition. Referring to vehicle manufacturer technical service bulletins (TSBs) can save a lot of time and frustration for these kinds of problems.

Something else that complicates diagnosis is that “standardized” OBD II codes really aren’t. There are actually two different types. “Generic” OBD II codes are the same in the sense that all vehicle manufacturers use the same code numbers to indicate the same type of problem. But each vehicle manufacturer also has their own special “enhanced” codes that cover problems not included in the basic OBD II code list. These include many problems not covered by the generic codes as well as problems that are outside the engine management system such as ABS codes, climate control codes, body codes, air bag codes, etc.

Generic OBD II codes all start with “P0” while the OEM enhanced codes all start with a “P1.” Enhanced codes are often vehicle specific and may not be readable with some code readers or scan tools. In other words, it may require special software or a dealer scan tool to read the enhanced codes.

A couple of things to keep in mind when buying replacement sensors: Parts that are physically interchangeable may not be calibrated the same and won’t work properly if installed in the wrong application. To make sure our customers get the correct replacement part, it may be necessary to refer to the vehicle VIN as well as OEM numbers on the original part. Some aftermarket parts may not look exactly the same as the original. A “universal” O2 sensor, for example, may fit a large number of applications but usually requires cutting and splicing wires to install.

Though most O2 sensors have no recommended replacement interval (replace “as needed” only), sluggish O2 sensors can be replaced to restore like-new performance. Unheated one- or two-wire O2 sensors on 1976 through early 1990s applications can be replaced every 30,000 to 50,000 miles. Heated three- and four-wire O2 sensors on mid-1980s through mid-1990s applications can be changed every 60,000 to 80,000 miles. And on OBD II equipped vehicles, the sensor can be replaced once it has seen 100,000 miles or more to restore like-new performance.

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