LH 2.4, 3-wire probe.

I would appreciate some opinions on the following readings:

Resistance cold - 4 Ohms, resistance hot 5.6 Ohms

At hot idle, the voltage cycles between 0.2V to 0.8V, at the rate of about one reading every half second. It couldn't be called "rapid", as it is very easy to see all the values, which cover the whole range. The "average" reading is definitely above 0.5V. In fact 0.6V to 0.7V would be about typical.

On blipping the throttle hard it rises to 0.8V, and then slowly sinks back to the idling pattern, taking a second or two.

Would these figures point to a "tired" sensor? It is still the original sensor (1993) on 320,000 miles. Apart from these readings the car runs well, and returns quite a good miles per gallon (32-36 UK mpg), but it only just passed the recent test - all gases were up at the top of the permitted range.

Many thanks

Cliff Pope

On blipping the throttle hard it increases to about

I recently replaced an O2 sensor on one of my cars. At idle the old sensor moved a bit quicker than this inasmuch as it was difficult with a digital meter to see each of the numbers. They changed too quickly. The Haynes manual states "the numbers will flash very quickly, so be observant".
Apparently, yours is moving slower than this. Also, I believe it should react quicker on throttle to idle transition.

As for the voltages, these seem ok. The Bosch FI book shows Lambda 1.00 = about 0.7 for a new sensor and about 0.6 for an aged sensor.

As for the heater resistance, the Bentley book and the Haynes indicates cold heater resistance = 3 ohms; hot = 13 ohms.

As for mileage, references point to about 100k (or less) for a maintenance change interval.

It seems to me that more than likely the sensor is operating lazy. On the other hand, it is still performing and has not set an ECU fault code. These sensors seem to last a long time. Nonetheless, if it was my car I would probably change it, if not for anything else, for mileage. Also, to help prolong the life of the Catalytic Converter. I would suspect that the mpg would improve (eventhough it's already good) slightly.

If you want to conduct additional tests, there is a lean and a rich test that can be done. The FAQ has details.


--
dnvolvo '89 765T 200K - '91 245 100K

Many thanks everyone. Yes, I'll be leaving well alone for the moment - I've got a year to worry about it in, so no need to do it all now!
So the Haynes statement that the voltage will fluctuate rapidly so watch carefully, is a bit misleading, given that you say the real underlying fluctuation is actually too rapid to pick up on an ordinary digital voltmeter.

I just tried it again, having got the engine really hot climbing a hill. The hot resistance had come up a bit, to 8 Ohms, I so I guesss that is OK, although not the 13 Haynes says.
But I didn't get any voltage alterations this time on reving the engine - it simply kept going through its idling cycle, all readings in about one second intervals from 0.1 to 0.9V. Engine speed made no difference.
I tried pulling off the vacuum pipe to the flame trap and it hesitated and voltage fell to 0.1 (ie weak). Then when I put my finger over the hole it rose to 0.9 (ie rich). So I suppose that means it can detect mixture variations correctly.

Hi Cliff,

I will tell you from my experience, you cannot evaluate a slow or lazy sensor accurately with a digital voltmeter. Even with a very fast sampling meter, with a rapid update rate, you are trying to get high-definition TV from a hotel lobby security camera.

Good for you, to get readings of the heater and an injection system in closed loop. The fact you are consuming fuel at an expected rate and satisfying the current emission testers makes me think you will leave the sensor alone until next time, using those readings as a benchmark.

The heated sensors do indeed last a long time, much more dependent on whether it gets poisoned by improper engine operation or contaminants than mileage alone. But 320K might be a long time to have anything threaded into an exhaust system and come out easily. Still, I say, cross that bridge when you need to.

BTW, my first impression is the hot reading on the heater was a bit lower than I remember; but that is mostly dependent on how quickly you get the meter in place after disconnecting the power source. Doesn't matter anyway-- the fact it works (hot and cold readings different) is evident.
--
Art Benstein near Baltimore

Have to agree with everything Art says. If you really want to check that sensors abilities you need an oscilloscope and a propane enrichment kit. You need to see that it can reach .9 volts and drop down to .1, you need to know that it can go from low to high in 100 milliseconds or less and you need to see one full cycle, low to hig and back to low, in 300 milliseconds or less. With the scope hooked up you need to feed it a little (damn little or it will stall) propane and watch the voltage peg, then keeping it pegged shut off the gas and see how fast it drops to .1 and then snap the throttle and watch how fast it goes from low to high, this is where you need to see a time frame of less than 100 milliseconds. This is, more or less, the approved method for checking O2 sensors for the state of California.

Mark