Oxygen sensor investigation

Blue Print Technical Consultant Jim Gilmour, looks at 4 Wire Wide Band Oxygen Sensors

Published:  04 February, 2014

By Jim Gilmour

It's when the sensor is 'on its way out' that complicated problems arise.

The DTC's and data that we see can be very difficult to interpret, and very often the sensor is changed to eliminate potentially incorrect information from a 'dodgy' sensor. However, we all know this is not best practice and it's an injustice to pass on the cost of a sensor when its replacement has no effect on the problem.

Recently a technician found himself in a situation where he replaced the front oxygen sensor on a 2009 Toyota RAV 4 with a running fault. This decision was based on the strength of P0171 code and a quick test with a voltmeter, which gave 'odd' readings. The sensor cost £175 +Vat and had made no difference to the poor performance. The odd reading of 0.3V was taken across the sensor wires and never changed, whether the engine was running or not. The new sensor gave the same reading, so he contacted Blue Print for some help and advice.

So how does it work?

So what is going on?

Figure 1

On Toyotas when the engine is running, at Lambda 1 the sensor produces a voltage of 0.3V measured across the sensor. The Electrode B exposed to the atmosphere (air reference chamber - normally negative) is supplied with 3V from the ECM - Electrode A consequently has a voltage of 3.3V (3V + 0.3V). The job of the PID Driver in the ECM is to maintain the 0.3V difference no matter what the oxygen level in the exhaust.

At stoichiometric (Lambda 1), the voltage at Electrode A is 3.3V and is connected to the Op amp the other input to the Op amp is a fixed 3.3V. The Op amp outputs the difference in voltage between the two inputs. The job of the PID Driver is to maintain the voltage of Electrode A at 3.3V. So at Lambda 1 the voltage output of the Op amp is 0 consequently the PID Driver outputs 3.3V to balance the voltage at Electrode A and no current flows across the resistor.

When the engine is running weak the voltage at Electrode A drops. The Op amp then outputs a positive voltage and the PID Driver reacts by increasing the output voltage supplying Electrode A, which raises it back to 3.3V and results in a current flow across the resistor. The weaker the mixture, the greater the PID Driver's output voltage, which therefore increases the current flow across the resistor.

When the engine runs rich the opposite happens; the voltage at Electrode A increases and the Op amp outputs a negative voltage. The PID Driver in turn lowers its output voltage causing current to flow from Electrode A to drain through the PID Driver maintaining the 3.3V at Electrode A.

Why do we use them?

how

AFR sensor problems

AFR sensors suffer from the same problems as narrow band O2 sensors

Testing the sensor

Nissan, Toyota, Mazda and some other Asian manufacturers have active test software built into their PCM that allows the technician to alter the fuel injection quantity, in order to produce rich and weak mixtures.

Normal mixture compensation is switched off during the active test and the system reverts to normal fueling when the test is cancelled.

Note:

Using injection quantity

The diagnostic tool used to carry out the Actuation Test is Blue Print's G-Scan - the G-Scan allows you to carry out the test on the fuel injection system, whilst viewing the resultant changes in voltage provided to the sensor.

If your scan tool doesn't have this function, the sensor can be tested in the following way:

Make sure the engine coolant temperature is over 80C

So in conclusion...

It's more likely that the fault is not caused by a faulty sensor so check all the usual suspects

For more information on Blue Print's G-Scan, simply visit www.blue-print.com/gscan

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