P0402 and P0155 on 1999 Cougar

dpfe sensor and/or plugged ports in the throttle body and passageways for the egr system and assuming the wiring is good likely an 02 sensor, unless the egr problem is throwing the 02 out far enough to generate that code. Pretty common failures on Fords, lots of posts on the forums about dpfe's and Fords. Probably worth giving them a read.

Since the time of my original post, I am now only receiving the P0155 and P1151 codes. The P0402 code went away without a code reset. The description of the remaining codes in the Motorcraft manual is as follows: P0155 HO2S Bank 2 Front voltage/current fault P1151 Front HO2S Signal, Lack of switching, sensor indicates lean, Bank 2 It also states that "lack of switching" malfunctions may be caused by HO2S sensor malfunctions or shifts in the fuel system and that DTCs are stored that provide additional information for the "lack of switching" malfunction. Based on this information I am lead to believe that the Front HO2S sensor on bank 2 is faulty. I am looking for concurrence, for as yet, having had 4 (and still owning 3) Ford/Mercury products that utilize HO2S sensors, I have never had a case where the sensor was actually bad. Discover more Engine Ram Pickup engine The first thing auto part stores want to sell you is a HO2S sensor for anything remotely smelling of an emissions problem. I have never bought one and have always found another cause. However, this time I am almost convinced it must be the HO2S sensor. Best Regards, Ted

i'm not! why would you think the dtc for an egr problem wouldn't be causing the lean codes!

The reason I was thinking that the sensor might actually be at fault this time is because the description of the P0155 code in the Motorcraft manual reads as follows: The HO2S heaters are monitored for proper voltage and current. A HO2S heater voltage fault is determined by turning the heater on and off and looking for corresponding voltage change in the heater output driver circuit in the PCM. A separate current-monitoring circuit monitors heater current once per driving cycle. The heater current is actually sampled three times. If the current value for two of the three samples falls below a calibratable threshold, the heater is assumed to be degraded or malfunctioning. (Multiple samples are taken for protection against noise on the heater current circuit.) I was thinking that since this particular fault appears to be related specifically to tests of the sensor itself, that there would be a problem with the heater in the sensor or the wiring going to/from the sensor to the PCM. Is this not correct? Thanks, Ted

maybe! if it were only a heater code I'd say sure but the not switching dtc p1151 means it could be staying lean, therefore the 02 signal could be valid and something is causing the the exhaust gases to stay lean! I guess you could try replacing that 02 first and see if the dtc po402 stays gone and the new 02 starts to switch as it should!

Thanks for all the input Jeff. Do you happen to know if there is a fairly straightforward way to test the O2 sensor via a Volt Ohmeter? Maybe check resistance/voltage differential, etc.? If not, I wonder if it would be worth the effort (and potential risk of damage) to swap the upstream O2 sensor where the errors are being reported with the corresponding O2 sensor in the other exhaust downpipe to see if the problem follows the O2 sensor or continues to be reported from the same bank of cylinders, which would indicate the O2 sensor was not at fault (or at least not the entire problem)? Thanks, Ted

downstream and upstream are often not the same so it's very unlikely they'll plug into the same connectors and if so it's unlikely they'll work the same! if you're gonna remove it replace it! Taking theings out and switching them around only adds to the risk of doing more damage! An ohm meter is pretty much uselss for testing an 02 sensor, Unless you have a graphing dvom sticking the voltage leads to the signal wire on the 02 will only shopw you the 2 ends of the scale of the voltages that it produces, not cross counts etc! That's the job of a scope. Like I said start with an 02 and keep us postted

Since my last update I noticed that the connector for the upstream O2 sensor on bank 2 to the wiring harness was significantly separated. I pulled it the rest of the way apart to check it for corrosion, etc. and then plugged it back in and cleared the codes (disconnected the battery). I drove the vehicle for approximately 15 minutes and the code did not come back. I then drove it for 15 minutes or so after it had set for an hour and a half and the CEL came back on. The same codes were present again (P0155 and p1151). I replaced the upstream O2 sensor on bank 2 and unfortunately have not yet been able to test it because something in the clutch hydraulics went South after I purchased the O2 sensor and I haven't been able to drive the vehicle. When I get it rolling again I will update with the status of the O2 sensor replacement and the codes. Best Regards, Ted

Well Jeff, I got the clutch master cylinder replaced today (what I job). After driving the vehicle for a short test drive I let the car sit for about 30 minutes and then drove it about 3 miles and the CEL came on again. Of course, as you suspected, both the P0155 and P1151 error codes came back. This is the first O2 sensor I have every purchased and it was a waste. I haven't had much success locating a repair manual for a 1999 Cougar, although it appears there is a fair bit in common between it and the Mystique/Contour. I guess I need to locate the ECM and try to identify its input/output connections to the 4 pins in this O2 sensor connector so I can verify the wiring is good. It might be easier (and cheaper) to get Ford to diagnose this one. Any gut feel as to the possibility of it being the DPFE sensor as compared to plugged ports in the throttle body and/or blockages in the EGR passageways? Is there a fairly efficient/reliable way to "clear" these ports/passageways w/o tearing things apart? I wonder if Sea Foam might clean them out? Thanks, Ted

you jusy have to remove the throttle body, and all the hoses too it and clean the throttle blade and all the ports with either a pipe cleaner and some wd-40 or a little bit of intake cleaner and some compressed air. The Dpfe sensors on fords cause all kinds of codes when they go bad and I magine if the car was driven long enough you'd have the p402 code return. You could try getting an http://www.alldatadiy . subscription if you're having a hard time finding a manual but Autozone should be able to source you a chilton or haynes manual for your car. I've never tried autozone's website with there z-net system but maybe it'll give you some pictures. A new 02 sensor is hardly a waste on that car. Probably 75% of 02 sensors fail or wear out and never generate an 02 code and unless you install the wrong one it should never make the car run worse or use less fuel so think of it as good maintenance! A failled egr valve itself I suppose can generate 02 readings that are weird as well but anyone that knows fords knows the dpfe's and throttle plugged are almost always the source of egr codes, not the valve. Are you sure there isn't a blown fuse somewhere for the 02 sensor circuit. I've seen cars come in with 02's unplugged that only generated the staying at centre type codes, no codes for heater failure, lean or rich etc!

I believe the problem is now solved. I checked the wiring harness connector where the suspect O2 sensor connects and found that with the ignition turned on node of the pins had voltage coming to the sensor. Prior to replacing the sensor I had found this connection almost completely disconnected but didn't check to confirm whether or not voltage was being supplied. One of the four pins did have a ground present. I then began tracing this harness as it headed down the side of the engine and into a junction with other wires that split, some heading down to the bottom side of the engine (this did not appear to be a potential route for this sensor) and others which headed back up along the front of the engine to the top under the edge of the intake manifold where the harness split again. I then noticed that there were two connectors towards stacked one above the other on a mounting plate and the top one was disconnected. At least one of the wire color schemes matched that of the wires feeding the bank 2/sensor 1 O2 sensor connector. I cleaned the connectors, reconnected them, verified that I had voltage coming on one of the pins at the O2 sensor connector, cleared the codes, and I now have driven about 30 miles without any issue. I haven't had it go this long without coming back on. I am pretty confident that resolved the issue. Jeff, thanks so much for your valuable insight. If anything further shows up I will let you know! I wish I had checked the connector with my Volt/Ohm meter sooner. Best Regards, Ted Allen