I have a 90 CRX D15B2
If the ECU looses oxygen sensor signal what does the ECU do to compansate? Will it just default to 450mV or somthing?

 

check engine light will come on and ECU should enrich mixture as a fault or 'limp home' mode. if ECU loses O2 sensor signal, it does not revert to anything, just see's a problem and illuminates light and fuel mileage will suffer.

Hope that helps...

 

most of the time when you're working with a dead O2 sensor. You car will usually default to run rich.