What To Do When PCMs Go Bad

Modern vehicles contain a stunning array of electronics. Sensors control everything from the way the automatic transmission shifts to the ratio of the fuel and air mixture hitting the combustion chamber. Each sensor takes continuous readings on the environmental factors that can affect the operation of the engine. The sensors’ corresponding control module adjusts parameters based on the readings. Continuous adjustments keep operational parameters within the correct ranges as different conditions occur. On a short trip down the block, thousands of signals transverse the wires connecting the various sensors and control modules to the car’s central computer: the powertrain control module (PCM).

The PCM is the brain of the car’s central nervous system. It is responsible for decoding the signals sent from sensors and taking the correct actions based on environmental factors. The PCM coordinates adjustments to the engine, so that it is all but seamless to the driver. Sensors located at various points on the automobile connect to the PCM through a tightly routed wiring harness. Car designers engineer the PCM system to be self-regulating, thereby eliminating the need for frequent adjustments to the engine to keep it running optimally.

The system works wonderfully, helping to keep vehicles running smoothly and efficiently. However, when something goes wrong with a car’s computer system, it can result in significant operational problems and high repair costs. Diagnosing vehicle computer problems is not always a straightforward process.

Determining a Faulty PCM

One problem faced when diagnosing a bad PCM is that other issues may mimic the same symptoms. For instance, a bad MAP sensor can cause the engine to idle roughly or die when taking off from a stop. These can also be symptoms of a PCM that is having trouble doing its job. Any sensor that connects to the main computer going bad can result in similar behavior as a failing PCM. Here are some common symptoms of a PCM or other part in the system that is either going bad or has completely failed.

  • Engine idles roughly
  • Degraded performance
  • Decrease in fuel efficiency
  • Vehicle will not start
  • Check Engine light blinks or stays illuminated

The fact that a variety of factors can cause these symptoms can make troubleshooting using traditional mechanic methods a nightmare. Tracing down the myriad of electronic signals that keep a modern engine correctly running requires assistance from computers designed to track and diagnose faulty signals.

Another possible issue can be a problem with the pathway on which the signals themselves travel. Frayed wires and worn connectors are a common problem with car computer systems. In this case, the old mechanic’s adage about “checking the simple things first” would apply. The trick to following this advice is accessing and checking the signal throughout the wiring system.

To complicate matters, the voltage levels from the battery can also cause symptoms similar to a bad PCM. The car’s computer, sensors and actuators are all susceptible to voltage levels. A battery shifting out of specifications due to it going bad can also wreak havoc on the electronics.

Troubleshooting a PCM Problem

While tracing and finding a computer problem using traditional troubleshooting methods can prove a near impossibility, using electronic diagnostics can be a relatively straightforward process. It takes the power and speed of a diagnostics computer to find problems with any degree of precision. Most qualified mechanics in properly outfitted shops can perform these tests and determine most issues.

However, these diagnostic tests can cost quite a bit of money. In some instances, car owners may be able to determine electronics problems thanks to the availability of inexpensive onboard diagnostics (OBD-II). Manufacturers equip most modern vehicles with OBD-II ports into which small diagnostic readers can plug. These readers are available from around $100 commercially. Also, many auto parts stores and mechanic shops will allow customers to use one of the devices free of charge.

Onboard diagnostics work by reading the code sent out by the PCM to detect a problem either with itself or another component in the engine. By understanding the codes and cross-referencing their meaning, owners can determine what part needs replacing. The readings from these devices are not 100 percent accurate, but they can at least help narrow down the problem area.

Hunting Down an Issue With Auto Electronics 

If the PCM is the main suspect, it is prudent to eliminate any copycat possibilities before replacing the unit. PCMs are expensive and usually non-returnable. Also, make sure the OEM part number matches the returned unit since many similar-looking modules are, in fact, operationally incompatible.

The troubleshooting procedure would follow a logical process to isolate the failing part.

  1. Locate the PCM – The powertrain control module may be located in several different places depending on the vehicle. Consult a manufacturer’s manual or aftermarket guide to find the unit.
  2. Inspect the PCM – Look for faulty wiring, cracked casing or damaged connectors on the unit.
  3. Find OBD-II Data Port – The OBD-II port is usually located somewhere under the dash. Consult the manufacturer’s manual for the exact location.
  4. Use OBD-II Scanner – Different scanners have widely varying operation instructions. Consult the manual for the proper testing procedure. Once the device reports a diagnostics code, cross-reference the code to determine the problem.
  5. Swap Part if Necessary – If the problem is a faulty PCM, the user must replace the unit with a new or reconditioned OEM or aftermarket part.

Finding the Right Part 

Purchasing replacement aftermarket or OEM PCMs can be risky. The unit must internally match the one it is replacing. Many units can appear identical but have significantly different programming, which causes them to be incompatible.

The expert used part specialists at Importapart.com can help make sure the replacement PCM is a perfect match. They have extensive experience with all brands and types of parts, so they know how to make sure the part will work in the desired application. Do not take a chance of getting the wrong part when dealing with something as expensive and non-returnable like a PCM. Trust Importapart.com to help find the perfect match.