O2 Sensor Question on 96 with OBD2

[gblanc]

Check engine light recently came on and had it scanned at Checker and it shows P0147 Oxygen Sensor #3 Heater Circuit Malfunction. This is the same code I had 10 months ago, at that time my shop replaced the rear O2 sensor on top of the cat. I noticed that all the O2 sensors seemed to be the same and most specify the same replacment sensor for the pre or post cat sensor so I swapped one of the front sensors with the suspect rear sensor, drove around for 20 min and rescanned. I got the same code.

This would indicate to me that either the problem was not the O2 sensor or that somehow the old code needed to be cleared first. It seems that all the info I can find on clearing codes is for 95 and older prior to OBD2. I am led to beleive that with ODB2 on 96 and later that the codes will clear when the problem is resolved.

Any thoughts ??

Any thoughts on how to test an O2 sensor, particularly the heater ?

[NiftySVX]

That code is detected when there is a malfunction in the heating element for the sensor. You can check for an open or shorted circuit across the heating element wires of the sensor. The computer also measures the current flow across the element and will throw a code if it is below or above the detection value. It would not surprise me if the sensors were all identical, with the eception of the length of the lead, but even that may be the same. Your best bet would be to start with a factory part to make sure that it isn't simply an aftermarket sensor that does not conform to the current specifications for the computer as new ones did. I deal with this all the time, the computers do not like to have any of the aftermarket sensors they sell at autozone and similar stores, especially the OBD II cars.

[Trevor]

Testing the operation of an O2 Sensor ©

To properly test a sensor under operating conditions, you will require a high impedance voltmeter, so that a significant load is not imposed on an output capable of supplying very little current. Most battery powered digital instruments are suitable.

It will probably be difficult to make connections at the connected plug/socket, with the engine running. Therefore locate a couple of sharp pins, the type with a large round head will be easiest to handle. A pin can be pushed completely through the insulation and wire, to make solid connection without significant damage..

To test the sensor operating conditions, access the the connecting wires, one wire, probably black, is ground. With the engine running, measure the voltage to engine ground on all three, as a means of identification. The ground wire will measure zero. The heater element wire, approximately battery voltage and the output wire, less than less than one volt.

Connect the meter permanently, in order to measure the voltage on the output wire. Run the engine up to normal temperature and rev up and down several times. Provided the engine is warm so that control is out of closed loop, the voltage should rise and fall rapidly around 0.02 - 0.7 volts. This will indicate a satisfactory sensor. The set point is 0.45 volts and you are expecting the reading to swing about this point.

To take things a little further, you can create a lean condition, by opening the power brake vacuum connection, or another such manifold inlet. This should drop the voltage to under 0.03. If you choke the engine, by partially blocking the air intake, so as to make it run rich, the voltage should rise to, 0.7 - 0.9

N.B. The control must be in open loop and its sensor element warmed by the inbuilt heating element, as when the engine is at proper operating temperature.