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.

Bob, I am in the exact same situation that you are. The only difference is that my car only shows:
P0171 (Generic) System Too Lean (Bank 1)

I went home today during lunch and disassembled the air intake and clean the Throttle Body. It has seemed to help but not much.

I also get Failed O2 tests on Bank 2 but I don't know if this is because we don't have a Bank 2.

My EGRUPHDIS and EGRDNHDIS is at a value of 8.08 inches of water with both failing.

I was going to attempt to clean the EGR Valve this weekend to see if that would help my situation.

The only other thing in the Mode $06 is that the
"Missfire Event Monitor Minimum Complete is 31211 counts with maximum of 0 has FAILED"

I really don't understand this one because it doesn't FAIL any of the other Missfire tests...

I would in your case replace your EGR Valve first. Then reset the computer and see how it reacts after a few days of driving. You can get our EGR Valve from Fordpartsonline.com for 64.40, The O2 Sensor is 65.69

I found this link about EGR that would be relevant:
EGR Operation and Diagnosis

You can also test the O2 sensor ? :
O2 Sensor FAQ

Dave

Curious I'm not familiar with engine computers but are/can these codes generated without CEL's or is there always a CEL when a code is produced?

All of the time when you get a CEL or MIL as the Technicians call them they have a Code that is present. Some cars will have a Service or Maintence Required light(Honda) for filter changes and the like which aren't really CEL's. In those cases there wouldn't be a code present.

Does anyone have any intimate knowledge of the EGR system who could help us out?

  This evening, I was running the OBD-2 software, watching a replay of my drive home, with both of the O2 sensors being graphed.  Most of the time, both O2 sensors seem to show normal activity, with sensor 1 showing, perhaps, a lower voltage level than usual.  From time to time, however, both sensors, in unison, would drop to zero and stay there for a short while, before resuming normal activity.  When this happens, the SYS STAT item on the DATA display switches from the normal closed loop mode to ?Open loop due to detected system fault?, returning to closed loop shortly after the O2 sensors resume normal behavior.  On this particular journey, I only got one PTC, which was P1131.  It occurred right after one of these instances where the two O2 sensors dropped to zero.  Apparently, the PCM doesn't see a need to generate a trouble code every time this occurs.

  I wonder if I'm looking at something as simple as a loose or dirty conenctor somewhere.  Is it possible that there is, at some point, a single connector, where both O2 sensors are grounded through a common pin?

Nebaukanezzer wrote:

?are/can these codes generated without CEL's or is there always a CEL when a code is produced?

  I've seen conflicting acocunts of just what the rules are, but apparently, there are some faults which are regarded as serious enough that the CEL will light up as soon as one is detected, but most faults are considered much less severe; for these, when such a fault first occurs, a Pending Trouble Code (PTC) is set, which can be retrieved via an OBD?II scanner, but which does not light the CEL.  After some amount of time, or some number of siuccessful drive cyles, if the fault is not repeated, the PTC clears, and the driver will never have known that it had occurred unless he had retrieved it via an OBD?II scanner.  If, while the PTC is still active, the fault occurs again, then the PTC becomes a Diagnostic Trouble Code (DTC), and the CEL lights up.

Bob:

For a 96 Zetec, I would guess the car mileage is approaching 70K miles, more or less?

I think you had the answer right under your nose too close for you to see?



My guess is that given the mileage, the O2 sensor eventually got contaminated with lead, carbon, metals from motor oil additives, and silica (from high-volatile RTV silicone sealants or anti-freeze) in the exhaust. The contaminations would have coated the sensor's platinum, rendering it sluggish with time. (This said, a poor-man O2 sensor rebuilt can be achieved by removing these coatings by soaking the tip in solvent such as gasoline overnight, and blow dry it in the morning?? It is also assumed that gasoline as solvent does not also *remove* the thin precious platinum coating, that may be at the end of its life cycle after 70K miles!)

It is worth noted that the PTC was set during closed loop, the engine was warmed up and the sensor was heated, i.e. the sensor was up and running for a while. Then "from time to time" both sensors dropped dead momentarily!? It may be coincidental and sensational that both need to be replaced, i.e. aged at the same rate, but when they're gone, they're gone !

However, you can wait until the PTC becomes DTC, as you seem to have the tool to play with them for a little longer. It would be less of a game when it is over?

I found this link provides more info on the O2 sensor (not again !)
Oxygen Sensor: Operation and Testing

Dave

There is another possibility. On the 95-97 cars there is a Underhood Wiring Harness recall. You might have a short due to faulty wiring which is causing both the lower voltage and the sensor drop off. I just had mine replaced since the Positive Battery Terminal had lost all of its Insultaion and wasn't charging the battery. There is wiring that has been replaced near the O2 Sensor's Connector from looking at the work they performed. Just a thought.

My CEL said the sensor was 'reading low'. Could this be the problem (the short)? It also said the valve is stuck open.....(see post in problem forum).