Standalone in Parallel with stock ECU

[tmontague]

Is anyone on this forum running a stand alone ecu in parallel with the stock ecu? Meaning hvac, dash and everything else is still working. Does the OBD II still work in terms of showing readiness signal (for passing e test) and do any CEL's pop up?

How do you get the stand alone to over run the stock ECU to run things like timing and fuel if both ecu's are connected? If you disconnect the stock ECU from the timing and fuel components does that throw CEL's?

I'm trying to get a better idea if this is possible in a etest province