"ECM"             Engine Control Module

"PCM"             Power-train Control Module

"ECU"             Engine Control Unit

"Brain Box"     Engine Control Unit

They all mean same thing. Ninty Percent of the time, these ECM modules can be found inside of the vehicle's interior compartment. Ten percent of the time, vehicles will have the ECM close to the firewall or next to the battery in the engine bay. Here is a interesting video about a GM vehicle having a ECM issue. 

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