Diagnosing ECM PCM failure by ECMToGO.com Auto Computer Pro



Many people are surprised to find out they have a computer in their car! Many newer models may have more than seven independent and cooperative computer controller devices aiding in the performance, comfort and environmental factors. They are even more astounded when they find out that the cost is anywhere from $650 to $2000 for a new unit from the dealership! To understand the ECM we can help by a short history of the ECM and implementation of computer controlled electronic systems. The Federal Government implemented the use of computer controlled ignition and fuel systems for 3 main reasons:

  1. To aid in the safety of the consumer
  2. To better economize the fuel consumption
  3. To reduce the output of EPA regulated byproducts of the ignition controlled internal combustion gas/diesel fueled engines in passenger and commercial vehicles

Often when your computer is failing there is misdiagnosis and people will spend hundreds I’ve even seen customers who have unfortunately paid over a $1000 on their car on repairs and part replacement before he finally got to us and he found out he needed only an ECM to solve his problem. I have seen people go without their vehicles for months because the mechanic could not find out he had a computer failure. On the other hand there have been countless others who have paid this amount and more on new computer ECU(s) and the problem does not go away! The key is proper diagnosis! My father always said measure twice cut once. So we are firm believers in getting a second opinion or third if necessary to collaborate repair diagnosis that can be financially crippling.

Most common misdiagnosed parts are battery, distributor, ISC motor, TPS switch, injectors & intake cam sensor, and other sensors or bus system . If the technician does not figure it is the ECM they may sell you parts you may not need.

When diagnosing an EFI or electronic fuel injection system vehicle, here are the “inside” industry approach procedures to engine computer, or otherwise diagnosing a malfunctioning vehicle. These systems all require the same basic elements, and by checking all individually, you may be able to rule out and systematically diagnose your problem and save hundreds of dollars.


Is your car starting and running, but stalls or seems to not idle right.

  • If so, is the check engine light illuminated MIL or (malfunction indicator lamp) on intermittent or all the time?
  • If so, what are the MIL DTC diagnosis codes? Try pulling ECM and looking and smelling for burns. Have your ECM tested to be sure it is functioning properly. If the ECM is bad, have it repaired or replaced with known remand or new unit. see www.ECMtogo.com
  • If ECM is OK, then follow the error codes it gives, and check the appropriate malfunction (i.e.: check engine light coded you had bad oxygen sensor, try unplugging it and restarting car to see if any change in running or the MIL. If no change, try putting a known good used unit in, or buy new unit if in your budget.)
  • Have ECM scanned in car by authorized tech with proper scan tools specific to application (only after installing or verifying you have a good ECM). Most manufactures have specific tools to their equipment even though there are many scan tools that can do basic diagnosis. This scan should show what elements are giving you the problems. There are some ways to check error codes without scan tool. Refer to manual for key to specific application.
  • This insider tip is for Mitsubishi & Chrysler Vehicles and is quite helpful for diagnosing or testing your ECM. You need an analog volt/ohmmeter, put the testers on pin 1 and pin 12 (top right & bottom left pin) of diagnosis ECM scan cable harness. This harness is inside the cab area, usually in driver´s side upper left of drivers kick panel. The sweeping motion left to right means ECM is OK. Sweep right to left means reverse testers to pin 12 and pin 1. A movement to the right without bounce or return means bad ECM.
  • Many models also have ways to check the MIL codes an indicator light on the dash or ecm or will illuminate the check engine light in series indicating the first digit then the second digit will follow and the indicators will repeat.

Your car Is not starting? As we started earlier, all efi engines need the basic elements to run:

  1. Battery power send voltage to starter to crank over the engine, and voltage references to and from ECM computer and supply power to relative components for vehicle operation
  2. Starter/solenoid
  3. Injector pulse- voltage reference from ecm to injector(s)
  4. Fuel (ecm sends voltage reference often via mpi relay to fuel pump)
  5. Spark to spark plugs to ignite the fuel/oxygen mixture for usable power

Knowing this you can then systematically check these separately…..

  • Power- battery/alternator system:
    1. If you have lights, you may be OK for cranking. But if she’s sat up for a while, you may want to have a battery charger or power/booster charger available and attached to your battery with a good grounded connection.
    2. If no lights check battery voltage with multi-meter. If less than 12 volts check alternator.
    3. If alternator bad replace, else replace battery.
    4. If you hear clicking and have power, check starter/solenoid. (Try tapping on the starter with a wrench a couple times. This sometime can be a temporary fix to free up the locked solenoid which often locks and won’t engage. Replace Starter or solenoid with known good remand or OEM model..
  • Injector pulse: Put a node light or meter on injector #1 to see that the ECM is sending injector pulse. If not pull ECM and look and smell for burns.
  • Spark to plugs: Check for spark at plugs, check the following if applies to your vehicle: distributor (coil, igniter, crank angle sensor, cam sensor), relay, ecu
  • Fuel/fuel pump reference
    1. Check voltage reference from ECM or from mpi relay to fuel pump. You can often hear it kick in. You can also see the flow from pump to ensure it is solid. Be sure to do this in controlled environment! IF fuel pressure is normal, it will shoot across the room! Have a volatile fuel receptacle ready like a larger glass jar or gas can
    2. If you have no voltage reference to fuel pump, check relay. If relay OK, check ECM for burns or smells. Test ECM.
    3. If reference to fuel pump but no fuel pressure, check fuel pump.

Information written and published by John Brain South c1997 ECMtoGo.com – Auto Computer Supply – Auto Computer Pro – Mater Tech Electronics