Ecm/pcm processor
The ecm continuously monitors its internal circuits. This self–check insures that the ecm is functioning properly.
If a malfunction is detected, the ecm will set the appropriate dtc and illuminate the mil.
The two cpus, main and sub cpu inside the ecm, perform continuous mutual monitoring. If there is difference between outputs from the two cpus that deviates from standard level ranges, the ecm concludes that there is a fault and sets a dtc.
: read freeze frame data using the hand-held tester or the obd ii scan tool. Freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air–fuel ratio was lean or rich, etc. At the time of the malfunction.
Replace ecmTest yourself
A quick Toyota Corolla quiz: every answer comes with an explanation and a link to the manual page.