Anyone know what the voltages of the ECU oxygen sensor connection is just reading the voltage with the O2 sensor disconnected. I seem to remember the ECU was .5 volts. My wire voltage from the ECU is showing .2 volts. Might be time to swap out the ECU. ? Anybody like to tests their 740 ECU O2 wire connection with the O2 disconnected ? I'm having trouble dialing in my "flashing" test light. The O2 sensor tests fine with full voltage sweep..🔧🔧🔧

ECU side:
With O2 sensor disconnected, engine stone cold not running 0.554 V (steady)
With O2 sensor disconnected, engine fully warmed up and running 0.576 V (steady)

O2 sensor side:
With O2 sensor disconnected, engine fully warmed up and running 0.3V to 0.1V (swinging)

I can't test voltage when sensor connected because I already wrapped up both plugs with electrical tape to prevent water intrusion. Weirdly the CEL (or lambda light) doesn't light up during engine running scenario. Maybe the ECU uses another criteria for worn O2 sensor. Maybe the ECU knows I'm doing some testing I don't know. Temporarily disconnected O2 sensor doesn't effect idle running.

Regards,
Amarin

1994 940 Bosch NA

I rechecked the ECU O2 sensor wire with the Oxygen sensor disconnected after the engine cooled down for several hours.....ECU reading is back at .5 volts. Reading the ECU O2 sensor with the engine warmed changes the .5 volts reading due to the Oxygen sensor and ECU wire connection showing voltage swing is operating fine. I decided on replacing the operational (Cheap Brand) Oxygen sensor with a Bosch replacement. I always remember the Bosch O2 sensors worked the best in all my Saabs and Volvo.....plus they seem to last longer and respond quicker. I just glad I have several ECU's and other assorted know relays and AMM's to carry in my cars tool box. My AMM test light is back to flashing again, so it leads me to believe a Bosch Oxygen sensor will respond better than a cheaper aftermarket brand.

Hi,

I will tell you how little I know, about what, I’m talking about! (:-)

On the 240’s, that have what they called the Lambda System,of which, is supposedly to mean a perfect combustion ratio, the sensor only puts out a voltage when at operating temperature.
The exhaust stream will have a narrow stream of burned oxygen when compared to oxygen surrounding the sensor’s outer housing.
The .5 voltage is the mean voltage of being the supposed perfect moment of adjustment. Either side is rich to lean and it’s why it’s constantly moving up and down.
A good working sensor and ECU system will vary closer in the middle.
A worn or lazy sensor will swing slowly or not at all.
Most of the time when they have failed they fail lean with a very low and steady low voltage. A dead output is exactly what it means!

The same thing theory is used with an EGR sensor except it’s not so much a voltage output but a change in resistance output back to the ECU.
When the EGR operates, a temperature change occurs on the sensor side of the EGR.
The ECU program is looking for that change in resistance within a certain time frame after each command for it to function.
From my experience, A clogged EGR (rare) or the vacuum valve that turns the EGR can fail. This is because of a funky exhaust filter, on its top, gets plugged.

If the ECU does not see the change, a CEL will be triggered.
Now when the ECU operates, within this system, it effects the exhaust gases and the O2 sensor so along with these two things they are constantly tuning the engine!
It is really fast that one misfiring plug will shift mixtures and enough of a series of them will change even the timing.
Watching timing marks or the sensor voltage can tell you how bad that is!
This year car may or may not have adjustable timing capability. In most cases you use a light to find bad firing faults anyways!
More times than not, ignition faults cause most fuel system adjustments!
This is the reason, why the jury is still out, on “exotic spark plugs” causing things they shouldn’t!
In far far reach, the AMM can also cause weird things if and when they get aged.

I will continue to explain “my lack of knowledge” by saying the the .2 volts that you reading is nothing more that component “leakage” from the sensing circuits within the ECU.
That voltage can be seen only as a proof the the ECU is on, waiting to be nudged and the incoming signal is going to get compared to a program, of which, is even more out if my knowledge range!

So there you go, do with as you please to help get your fingers dipped into the wonderful world under the hood!
(:)

Phil

With the O2 sensor disconnected, that might be a less than clearly defined level, depending on how the circuitry was designed. So, the 0.2 V wouldn't have me bat an eyelid unless the O2 sensor was connected.

With the O2 sensor connected and ignition switched on, but engine off, that voltage should be a steady 0.5 V.

If the voltage swing happens according to what's written in this FAQ, I'd say it's OK.