Contour Enthusiasts Group Archives

  The car in question is a 1996 Mercury Mystique, with the 2.0-liter 4-cylinder "Zetec" engine.

  Lately, I get, on occasion, various mixtures of the following OBD-II trouble codes:

   P0131 (Generic)        Oxygen Sensor Circuit Low Voltage (bank 1, sensor 1)
   P0133 (Generic)        Oxygen Sensor Circuit Slow Response (bank 1, sensor 1)
   P0171 (Generic)        System Too Lean (Bank 1)
   P1131 (Ford-specific)  Lack Of HO2S11 Switches - Sensor Indicates Lean

  Also, the manufacturer-specific Mode $06 tests sometimes, but not always, give failing results to the The O2 sensor at Bank 1 Sensor 1.

  Based on what I have written so far, would everyone agree that by far the most likely cause is that my primary oxygen sensor is failing and needs to be replaced?

  But wait, there's more that I haven't told you.

  Some time ago, I started getting the code P0402, which means excessive EGR flow.  I pulled the EGR valve out, cleaned it up, tested it, and determined that it appeared to be working correctly.  I then tested the EGR Vacuum Regulator Solenoid, which controls the EGR valve, and determined that it was malfunctioning, so I replaced it.

  This seemed to cure that problem, but several weeks later, it returned, and I was again getting the P0402 code.  On further testing, I determined that the EGR valve itself was failing to consistently close all the way when it should, and that it was the EGR valve itself, and not some other component, which was failing.  I've formed the intent to replace the EGR valve, when I can afford a new one, but have not done so yet.

  But I stopped getting the EGR code some weeks ago.  I still get a very rough idle, some times, which I believe to be a symptom of the EGR valve not fully closing, but no P0402 code; although I'm not getting the code, I still believe that my EGR valve is faulty.

  The Mode $06 tests related to the EGR valve have always shown at least one failing result.  There are two tests in particular, named EGRUPHDIS and EGRDNHDIS; one of these must, in order to pass, always show a value of less than 4, while the other, to pass, must show a value of greater than 504.  Every time this test is run, EGRUPHDIS always shows the same value that EGRDNHDIS.  The value varies from one instance of running these tests to another, but each time, both tests return teh same value.  The value is always either greater than 504 or less than 4, so the result is always that one of these tests has passed and the other has failed.  The EGRSTUCK test sometimes shows a failing result, and sometimes shows a passing result.

  I've been told that a malfunctioning EGR valve can cause wonky readings on the O2 sensor.

  I guess I should mention that the P0402 code only ever occurred when the car was stationary and idling, while the codes related to the O2 sensor and to the mixture being lean have only occurred while the car is in motion.

  In the near future, I expect to be able to afford either a new EGR valve, or a new O2 sensor, but not both.  Which should I replace? The most obvious answer would be the O2 sensor, because the codes I am currently getting are related to it.  But maybe the EGR valve, though it is currently not failing badly enough to generate codes, is somehow screwing up the O2 readings, and causing a perfectly-fine O2 sensor to report bad results.