[Dalite]

I finally broke down and spent the bucks for a set of 2003 PT Cruiser Diagnostic manuals. Here is the description of the O2 Sensor's operation:

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O2 Sensor (NGC)

The O2 system with the ignition on and engine off has a normalized O2 voltage of around 5 volts as displayed on the DRBIII or measured with a high impedance voltmeter. As the O2 sensor starts generating a signal the voltage will move towards 2.5 volts. The voltage will typically vary between 2.5 volts and 3.5 volts on a normal running engine. The goal voltage is also typically between 2.5 and 3.5 volts. This implies that the 0-volt through 1-volt range that you are used to is shifted up by a 2.5 volt offset. This 2.5 volt supply is being delivered through the sensor return line.

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Here are some notes (from the manual and elsewhere)
Notes:

NCG = Next Generation COntroller. This is the term for the new PCM that integrates the Transmission Controller internally. It also introduces some new diagnostic prpocedures and new DTC numbers.

DTC=Diagnostic Trouble Code

DRBIII - Scan Tool / Diagnostic instrument used for NGC PCM diagnostics, and provides communication with the PCM (2 way; input and output).

High Impedance Voltmeter = The Ohms per volt specification is an indication of a voltmeter's usefullness in measuring low current voltage sources. The higher Ohms per Volt reading, the less likely the meter's internal resistance will "load" the circuit under test, thereby giving false readings. This is a carry over from analog meter movements, and not as prominent a concern with most modern digital meters.

Sensor Return Line = Normally considered a signal ground; this is the reference level against which the voltage is measured. In theory, a blocking device, such as a diode, could be used in the return line to provide a "tap" for the 0 to 1 volt levels that most A/F gauges are looking for, while blocking the 2.5 volt offset from the measuring device (A/F meter). But, then again, the blocking device must not "load" the circuit when connected to a measuring device (ie A/F meter). There may be room for development here..

Here is a basic wiring description of the 02 sensor connector:

4 pin connector

1..... Ground
2..... O2 Heater Control
3..... O2 Return
4..... O2 signal

Pins 1 and 2 heater circuit.
Pins 3 and 4 O2 signal circuit

Diode 1N915 or 1N4148 (possibilities)

I don't know if this is a working theory. If you were to determine the average O2 sensor voltage, and then try the tap, the project could be considered a success if the average O2 voltage did not show a decrease over average readings once the tap and A/F gauge was in place.

This is probably low on the list of usable information. Installing a normal 0 to 1 volt sensor for the A/F gauge would be a lot easier in the long run, but being able to use one of those heated O2 sensors that are already there should give a more stable reading.[:I]