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ECU or O2 Sensor
Posted by Dave Wysocki (more from Dave Wysocki) on Tue, 6 Aug 2002 15:44:39
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

I've had an intermittent problem with my '93 9000 CSE T (5spd, w/TCS),
where within a few days after resetting any engine codes (or disconnecting
the battery), the check engine light will come on, with a flash code of 6
(o2 sensor error), or occasionally 7 (fuel-air mixture error).  The local
Saab dealer agrees that the problem is not with the o2 sensor itself (I've
tried 3 in the last year), but says that there is a 450mV reference signal
missing from the ECU to the o2 sensor, and that this reference level
'starts the sensor'.  My understanding of the Bosch 3 wire sensors is that
they are output only devices (except for the heating element), and although
I don't have a schematic of the ECU itself, it would make sense for the ECU
software to use a software value of 450mV until the o2 sensor is up to
operating temperature, but it wouldn't make a lot of sense to have a
hardware reference voltage put on the o2 input to the ECU.  When the system
is functioning correctly, the o2 signal toggles from about 0.2VDC - 0.8VDC,
but when the check engine light comes on, it stays at about 0.8VDC.

Thanks in advance for any suggestions.

Dave Wysocki
'93 9000 CSE T, 212K miles
'84 900 S, 420K miles, RIP
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