ecu hold codes or trip codes?

[dreemin4life]

I have a 94 ranger 4 cyl. with 5 spd. and i've posted before about having problems with the cel. My question is that I just replaced the pcm (ecu) because i was told it was bad and not the sensor it kept tripping. So i replaced it and the cel is still on. My question is, is it the pcm that trips the codes or does the system checks?? I'm gonna call the shop i took it too but wanted to know before i talked to them. Thanks guys.

Brandon

[AlfredB18]

If the computer detects a sensor reading out of the given parameters, then you get a CEL.

PCM is responsible for the system checks....