O2 Sensor Diagnostic

on Thursday 15 November 2007
in A-Z Index - Everthing is listed in here > O2 Sensors

O2 sensors monitor the amount of oxygen present in the exhaust gasses and adjust fuel mixture (slightly) within preset maps to give the best trade between performance and economy. O2 sensors do fail from time to time and can result in poor fuel economy (not exclusively, just one of many causes!) They are located in the exhaust manifolds just after the turbos (on a TT, present on NA's as well).

How can you test them?

all you need is a safety pin. Once you find the connectors - both are three pin connectors, with black, black/red, and white wires. Passenger side is near the rear of the passenger side plenum chamber, and driver's side is on the side of the engine, just in front of the master cylinder.

Check that the connectors are clean and free of corrosion first. Be careful undoing the connectors, as the rubber seal can drop out into the engine bay, never to be seen again.

Warm the car up, and with the car at hot idle and with the connectors all plugged in, insert the open safety pin into the end of the connector along the red wire, making sure that it makes contact with the red wire inside the connector. Connect the voltmeter (low volts range, eg 20 V) with red probe to the safety pin and black to earth. You should see 12 volts DC. Note the engine should be above 2,000rpm while performing the test.

Then plug the safety pin into the white wire, making sure that it makes contact with the white wire inside the connector. Connect the voltmeter (low volts range, eg 2 volts) with red probe to the safety pin and black to earth. A good 02 sensor when warm should show the voltage moving between around 0.3 volts DC and around 0.7 volts DC - this is the ECU cycling the oxygen content (via varying the fuel content) to keep the ratio correct. At higher revs, the upper voltage increases from memory, but it always cycles, except possibly at WOT (which I recommend that you DON'T do while parked in the driveway! ).

If the voltage reading doesn't cycle, then it's probable that the 02 sensor or cable/connector is stuffed. I'm not sure about the Zed, but some/most 02 sensors take a while to heat up, so until the engine and exhaust are hot enough, the ecu usually operates in 'open loop', which means that the voltage won't vary. Thus, you have to do the test when the engine is hot, eg after a 10 minute drive.

The Black/red wire is a heating wire I think - most 02 sensors have a heating coil in them to help them heat up. NOTE! I've seen a Zed 02 sensor test article on a web page somewhere which says to check the resistance between the red/black wire and the black wire - this is only testing the heater coil, not the actual 02 sensor, so it's not a valid test.

Lastly, the Zed ecu controls the 02 sensors via the black wires to each sensor. THEY DO NOT GO STARIGHT TO EARTH. Don't use the black wires for the above test - use a chassis earth, or the battery negative.

Lastly # 2. Last time I priced them, Nissan sensors were around $260 and $290 each (different lead lengths), and I bought an aftermarket one from Repco for around $110, but it has no connector.

Lastly # 3. The ECU diagnostic and the Consult DO NOT LOG 02 SENSOR FAILURES! They do show however if the sensor readings are cycling (led flashing on the ecu, and direct voltage readout on the consult), so you can still detect if they're working or not.

- Dangerous

While what everyone has said thus far is quite feasible, it is by no means diagnostically exhaustive with regards to O2 sensor operation to simply do the voltage check and/or flashing of the ECU diagnostic light and then draw relevant assumptions.

The main reason for this is that whilst the voltage should cycle under normal closed loop operating conditions, there are other conditions other than a faulty sensor which may cause the voltage to appear stationary or sluggish.

For example, if an injector on the L/H bank is faulty and is leaking, then a fully operational 02 sensor should detect this and the voltage will read at a stationary or sluggish level towards the high end of the 0-1.5 volt spectrum. The R/H sensor will alternatively be switching correctly. This may give the false impression that the L/H sensor is faulty.

Conversely if there is a blocked injector the reverse would apply and the voltage coming from the sensor (remember, O2 sensors induce their own voltage when heated. In this way it is possible to do the test when the sensor is unplugged) would be consistently low. The same rule applies here with regards to diagnosis.

Finally other things such as a faulty AFM or CTS could cause the sensor to read a stationary or sluggish voltage, though this should be easier to detect as both sensors will be reading consistently with each other.

Consequently if you do the voltage test under the appropriate conditions and both sensors are switching then you are fine. If however the voltages of either sensor do not switch then do not automatically assume that the sensor is at fault. Get it checked up against a gas analyser to see if the sensor is telling the truth.

- reebo