Anybody know where the leads connect in a 1994 E420? What (and where) is the small box that you speak of. And do you mean to totally disconnect the O2 sensor and measure the resistance between it and ground, or just pull up the leads enough to stick the meter in parallel?

Leave the O2 sensor itself alone. Follow the lead from the sensor to the black box and disconnect the lead there to do the measurements described. HOWEVER: that procedure concerned my former, 1991 200E (4cyl.), and I'm would very surprised if your V8 is the same. Better look somewhere else.

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2011 Prius

Before you all go buying O2 sensors, I would suggest that the proper voltage output of a fully heated O2 sensor depends on the mixture it is sensing. Its goal is to sense and the systems goal is to achieve an average reading of Lambda or .45 volts. A good sensor will not likely ever stay at .45 volts. It is easily overwhelmed by mixture reading close to 1v at CO percentages of 1-6%. At 6% the engine starts to run bad. At 1% the engine is way out of emissions range.

The easy, cheating, way to evaluate the system is to disconnect the sensor portion if possible (leaving the heater hooked up - even if one must jumper it - many applications can't keep the O2 lit off without the heater). Place your volt meter across the sensor pair of wires so that you read the sensor output.

Find the wire going to the computer that carries the sensor signal and ground it. With the engine reved one should get 1v output from the sensor which your voltmeter is hooked to. Now take the sensor wire and place one hand on a wire connected to the sensor to computer input and place the other hand on a 12v source. This tricks the controller into leaning the system and the voltage reading at the sensor should go to close to zero.

Modern cars concern themselves with how fast the change of voltage can take place. This testing will only qualitatively address that criteria. It is a way to see if basic function is capable.

Most O2 sensor codes originate from static sensor output voltage over a period of time. if the mixture is farther rich or lean than the controllers capability to correct then it will stay at one of the end stops 1v for too rich and 0v for too lean. This is NOT a sensor problem.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Don’t want to hijack this thread, but do have a similar/associated problem and questions. Have just done a rather detailed new thread “Can O2 sensor be causing my poor idle problem??” that I’d much appreciate if the experts participating here could help me with.

Regards,
Neil (London)