Stuck in Open Loop

[Felix Miata]

Is there a FAQ that covers this? What are the most common component
failures that cause this (other than O2 sensor failure)?

I have two of Mazda's first attempts at production EFI, '84-5 RX-7's.
One stays in open loop continuously. The other goes rich when it should
go lean (slight off-throttle or steady cruise), and lean when it should
go rich (light acceleration in closed loop).

I've only swapped ECU & O2 sensor between the two so far without effect.
The too rich only had a broken thermostat stuck open, but a new one
changed nothing.
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Team OS/2
FIXx ADDRESSs TOo REPLYy.
Felix Miata