My '90 240 is having rough idle problems, so when I read a post about measuring oxygen sensor resistance, I checked the resistance on O.S. leads and got 2.7 ohms cold and 5.5 ohms hot. The post indicated it should reach 12ohms/hot. Does that mean my O.S. is shot?
Also, got .55 volt at +heater wire (ign. on or off and wire connected or not). The post said 12v. was correct. What could account for the difference? Thanks.

Since the operating temperature of the O2 sensor element is in the 600 to 900 degree farenheit range, you're going to notice some lower resistances than normal due to the time it takes from shutting off the engine and getting your multimeter connected (the temperature drops a few hundred degrees in a few seconds after the car is switched off). Unless you've got a friend who's ready to instantly probe the contacts just as you switch off the engine, it will be hard to get an accurate reading.

For optimal fuel economy, I'd advise changing your Oxygen Sensor every 60,000 miles or so (you can let it go to 100,000 if you do a lot of frequent highway driving and it hasn't been fouled/contaminated). A new sensor will easily pay for itself in the 2 to 5 mpg improvement that it offers. I would avoid the aftermarket Walker/etc sensors at all costs since they often die after about 2 years of operation. The Bosch sensors are factory OEM, and will give you a good 5 years or more (60,000 or more miles) of good reliable operation and optimal fuel effeciency.

The new factory-fit O2 sensors cost a bit more for our Volvos, due to the lower manufactured quantities (around $120 to $150). However, you can convert a Bosch 13913 ($40) to work just fine by splicing the wires to the pigtail of the old sensor you just removed. You must crimp and use shrink tubing, and do not solder the wires. The new O2 sensors receive their reference air-oxygen by slowly "breathing" down the braided wires, and thus eliminating the air port on the back of the sensor itself. If you really feal ambitious and want to have a plug-and-play solution to using a cheaper 13913 oxygen sensor, you can grab the mating connector from a scraped late model Ford F-150/Mustang in the salvage yards and build yourself an adapter to mate the 13913 to your factory harness.

God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 261k miles.

The test in the Bentley manual for the heater on the O2 sensor is 3 ohms cold and 13 ohms hot ( above 660 F) resistance between the two heater wire leads ( not the single ECU signal lead, it could damage the ECU). If you are only getting 5.5 ohms hot, then it soulds like the heater is weak/bad. On my 86 with the LH 2.2 system, there is no voltage at the heater+ terminal unless the car is running. If just turned to the ON position, there is no voltage. Don't know what it should be for the 90 LH 2.4 cars. But if you don't have system voltage when it is RUNNING, you would have to start working backwards to find out why. If the O2 sensor element itself is good, and puts out good voltage/signal, then even with a bad heater many times the car will run fine. The heater just heats up the element to get the sensor and ECU into closed loop operation sooner, and not let it drop out when coasting and at idle. If you have constant bad idle problems, the trouble may be elsewhere.
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Gary Gilliam Sumerduck VA, '94 940 na Regina 160k '86 240 190k

If just turned to the ON position, there is no (heater) voltage...

Good point, Gary. On both the LH 2.2 and 2.4, the heater voltage comes from the Fuel relay, which is not "picked" unless cranking or running. So 0.0v with key on is normal.

And, as you said, it's the sensor voltage output that's important. The heater is good to have, but is not a critical item.


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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.