So far as I can understand (and I have studied it a lot), our ECM (at least the '87/'88 1228079 - I doubt the previous ones were "smarter") uses only coolant temperature to determine whether it is in Open or Closed Loop. O2 reading only appears to be used for Rich/Lean indicator for the mixture control solenoid.

Code:
EAAA:   B6 32 BC            LEAAA   LDAA    L32BC ;	MIN TEMP START CLSD LOOP
EAAD:   D6 11                       LDAB    L0011 ;	CLCC 1
EAAF:   2A 03                       BPL     LEAB4 ;	BRANCH IF OPEN LOOP 
EAB1:   B6 32 BD                    LDAA    L32BD ;	MIN TEMP CONT CLSD LOOP
EAB4:   91 35               LEAB4   CMPA    L0035 ;	COOLANT TEMP
EAB6:   23 23                       BLS     LEADB

EADB:   96 11               LEADB   LDAA    L0011 ;	CLCC 1
EADD:   84 7F                       ANDA    #$007F ;	OPEN LOOP
EADF:   97 11                       STAA    L0011 ;	CLCC 1
EAE1:   7E EC C4                    JMP     LECC4


Where the thresholds are

Code:
32BC:   B9                          FCB     $B9 ;	MIN TEMP START CLSD LOOP
32BD:   C6                          FCB     $C6 ;	MIN TEMP CONT CLSD LOOP


and CLCC is

Code:
                            L0011   EQU    $0011 ;	CLCC 1
;	b0 = BLENDED ENRICHMENT FLAG                 1 = BLENDED
;	b1 = COLD START                              1 = COLD START
;	b2 = IDLE LOAD CONTROL WIDE OPEN THROTTLE    1 = WOT
;	b3 = WIDE OPEN THROTTLE                      1 = WOT
;	b4 = WIDE OPEN THROTTLE DIVERT FLAG          1 = DIVERT
;	b5 = O2 RICH/LEAN FLAG                       1 = RICH
;	b6 = NONVOLATILE MEMORY BOMBED               1 = BOMBED
;	b7 = CLOSED LOOP FLAG                        1 = CLOSED LOOP 


Basically that says that the engine coolant has to be reading roughly 52 degrees celcius (about 125 Fahrenheit) to go from Open Loop to Closed Loop. After that, engine coolant must maintain at least 45 degrees celcius (113 Fahrenheit) or it will drop back to Open Loop.

Unless you want to tweak the PROM to lower these thresholds, you're wasting money and time on putting in a heated O2 sensor for the purposes of getting to (and staying in) closed loop. I can sell you (speaking to the general audience) a tweaked PROM that would lower the temperature thresholds, but there is more than what I'm showing here in terms of parameters. There are a few minimum time parameters where no matter what the temperature is the ECM will remain in Open Loop. No one has yet asked for a PROM with these parameters tweaked so I don't know everything it would take to tweak this properly (yet).


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