Contour Enthusiasts Group Archives Forums General Troubleshooting po161 code O2 sensor but which of 4 help?

I have a 1996 GL with 80,000 miles cel on ran code came back as power train 161 o2 sensor heat something failure but I have 4 one is bad but which one is it I am perplexed help please car runs great in between weekly trips to garage, junk yard, and auto parts stores. just kidding not really monthly trips i mean

The sensor you got the code from is the one that is closest to the oil flter. It is in the exhaust manifold, just before the y-pipe (at the front of the engine). It's one of the "after car" (downstream) sensors, so its malfunction won't effect driveability of the car in any way. It's more of an emissions monitor than anything else.

It is really easy to replace that one. Your problem is one of three things: A) a faulty heater circuit in the O2 sensor itself (could be, fixed by replacing the sensor), B) a bad connection at the O2 connector (could be, can be fixed by cleaning the connector pins - FREE), or C) some sort of open or short in the wires themselves leading from the PCM to the O2 sensor (least likely and most difficult to track down).

Try B first, then A, then C if a new sensor doesn't fix the problem.

Hmmm. I assumed that a malfunction on an O2 sensor would drive the mixture control for that cylinder bank to full rich trim, right? (lean is bad news, and with the defective O2 circuit, it has no way to monitor combustion efficiency. Seems like the safe position would be full rich).

Agree that driveability remains unchanged, but mpg decreases, right? May be of more concern to the owner than just increased emmisions, especially with the "outta there!" gas prices we've been seeing lately!

Does the 96 have 4 sensors? I know the 95's have 2, and I thought someone said the 4 sensors started in 98 model year (?)

Maybe all the above is just BS. I'm still learning about all this myself. :rolleyes:

FWIW

Jab

Try this.


http://www.obdii.com/codes.asp

does have 4 sensors, tried cleaning connector and reset code ran no mil or cel for 25 miles then back on, going to try and found out more info on mystery mod and mil elimenators ? then decide if to go that route or by a new o2 sensor. thanks for the post they were all helpfull. as a last resort i may just put some black electrical tape over light to help block it out.

Excellant Site Big Jim. If you punch in the code P0161 it comes back as H02S22 (Heated Oxygen Sensor bank #2/sensor #2). These (#2 sensors) are called "catalyst monitor" sensors and are NOT used for fuel control. They just mainly tell you if your convertors are working correctly.

They can usually be identified by their blue connecters compared to the green/sometimes white front (#1) 02 connecters.

BTW on all Fords H02S 11 (bank 1/ sensor 1) is always on the side where #1 cylinder is.

Just to clear up a common misconception, the downstream (or rear) oxygen sensors ARE used for fuel control in a secondary manner in addition to their primary duty as catalyst monitors. As the upstream oxygen sensors age, they shift lean or rich in the way they meter fuel. The downstream sensors can detect this fuel shift and feed back a trim bias to the EEC to keep the fuel control and pre-cats happy.

Mr. Weiss,
If this is a misconception then it's still being taught as gospel in the Ford service manuals, Technical Service Messages/Bullitens & by their instructers. I would be interested in recieving this information by fax (if you would please) if possible. The following is an overview from the Powertrain Control Emissions Diagnostics manual (PCED). Thank You (fax317-831-9531).

2000 PCED OBDII SECTION 1: Description and Operation

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Catalyst Efficiency Monitor - Federal Test Procedure
The Federal Test Procedure Catalyst Monitor monitors the catalyst system for deterioration and illuminates the MIL when tailpipe emissions exceed the appropriate HC emission thresholds. It is called the FTP catalyst monitor because it must complete during a standard emission test (the Federal Test Procedure). This monitor relies on the front and rear heated oxygen sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst.

Many Low Emission California vehicles will monitor substantially less than the entire catalyst volume in order to meet the stringent catalyst monitoring malfunction thresholds. In many cases, only the front, light-off catalyst is monitored.

Front and rear HO2S switches are counted under specified closed loop fuel conditions. After the required number of front switches are obtained, a rear-to-front HO2S switch ratio is calculated. The switch ratio is compared against a threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. Inputs from the ECT or CHT (warmed up engine), IAT (not at extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP(at part throttle) are required to enable the Catalyst Efficiency Monitor.
The DTCs associated with this test are DTC P0420 (Bank 1) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL.NOTE:THE CATALYST MONITOR ON SOME EARLY OBD II VEHICLES (SOME 1994-1996 VEHICLES) WAS REFERRED TO AS THE "STEADY-STATE CATALYST MONITOR" AS OPPOSED TO THE "FTP CATALYST MONITOR" (DESCRIBED ABOVE) THAT IS MOST COMMON FOR VEHICLES BUILT AFTER 1996. BELOW IS A BRIEF DESCRIPTION OF THE STEADY-STATE CATALYST MONITOR: The Steady-State Catalyst Monitor performs a 20 second test during steady state rpm and load conditions. The Monitor transfers closed loop fuel control from the front to the rear O2 sensors. The Monitor then observes the switching frequency and compares it to a threshold frequency stored in an rpm/load table. A frequency higher than the maximum calibrated threshold indicates a malfunction.
The Catalyst Monitor DTCs can be categorized as follows:
Catalyst system efficiency below threshold (Bank 1) - P0420
Catalyst system efficiency below threshold (Bank 2) - P0430

Paul, I can't get into it in detail, but it's a feature called Fore-Aft Oxygen Sensor Control, or FAOSC. With a healthy pre-cat at a steady idle, the rear sensor reads about 0.6v, which is slightly rich. This makes sense because the cat brick is absorbing your excess O2. Now what happens if that point starts to drift, assuming in this case all the upstream fuel control pieces are working correctly? If it drifts lean, <=0.5v , it could either be the cat going bad, or a front O2 sensor shift. If it steadily drifts even more rich, >.6v , then the cat is obviously OK, so maybe the front O2 sensor is drifting rich. FAOSC logic isolates this shift, and then uses the rear O2 input to move the front O2 sensor lean/rich switch point back to stoichiometric.

thanks to everyone for your insights and thoughts lots of great advice!