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Old 07-26-2001, 12:31 PM   #1
Jon [in CT]
*** Banned ***
Member#: 2992
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Connecticut, USA
02 WRX Sedan

Default 2002 Impreza Front Air/Fuel Sensor

Their seems to be a difference between the front air/fuel sensor used in the 2002 WRX (DOHC) and the unit used in the 2002 Impreza non-turbos (SOHC). The non-turbos have two extra wires.

Ignoring heater, shielding/grounding and sensor power, here's the service manual description of the two sensor wires for the WRX:
Front Oxygen (A/F) sensor signal (+) Idle Volts = 2.8 - 3.2
Front Oxygen (A/F) sensor signal (-) Idle Volts = 2.4 - 2.7

Here's the description of the 4 sensor wires for the non-turbo:
Front Oxygen (A/F) sensor signal 1 Idle Volts = 2.8 - 3.2
Front Oxygen (A/F) sensor signal 2 Idle Volts = 2.4 - 2.7
Front Oxygen (A/F) sensor signal 3 Idle Volts = 0.2 - 4.9
Front Oxygen (A/F) sensor signal 4 Idle Volts = 0.2 - 4.9

The WRX ECU can issue a diagnostic trouble code P1134 - FRONT OXYGEN (A/F) SENSOR MICRO-COMPUTER PROBLEM, whose remedy is to replace the ECU. The non-turbo ECU has no similar DTC.

The Subaru Select Monitor can display, for the WRX, front sensor current (mA) and resistance (Ohms), but cannot display this info with the non-turbos.

I suspect that the sensors are the same but the non-turbo sensor pigtail/connector contains circuitry which performs the function, more or less, of the "sensor micro-computer" embedded within the WRX's ECU. I suspect that the WRX's front sensor current and resistance are intermediate results, enroute to the final result of a 0-5V signal representing the A/F ratio. I suspect that the circuitry in the non-turbo sensor's pigtail/connector is supplying this same 0-5V signal directly to the non-turbo ECU.

It seems, therefore, a possibility that a non-turbo A/F sensor could be swapped into the turbo (or at least its outboard circuitry) and then its extra 0-5V signal could be used to drive an Air/Fuel Ratio meter that would likely be very accurate.
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