how can you tell if the ecu goes into safe mode

 

The Check Engine Light / Malfunction Indicator Light will normally come on. In the case of a PM6 OBD0 ECU, IIRC, there is a red light on the ECU itself that will come one or flash. It will flash in a specific sequence. This sequence of flashes will tell you what problems the ECU is encountering.

ECU light flashing sequence.
SHORT flashes are single digits (1/2 second)
LONG flashes are 10's (1 second)

You may have multiple codes, if there are multiple codes there will be a slight pause between the sets of code flashes.

Ex
short...short...short Pause long...long...short...short...short

In this example you would have a code 3 & 23. Once you have figured out the numbers that your ECU is flashing you need to check them with a list of diagnostic codes. Click here for a list of OBD diagnostic trouble codes.