My 1999 Diamante is showing a P0170 code (Fuel Trim). With a CarChip, I have graphed Short Term Fuel Trim (STFT), and the Front 02 and Rear 02 sensors. See the graphs below.

The problem of high fuel trim usually comes from a vacuum leak or low fuel pressure. I have checked for obvious vacuum leaks and driveability is fine so I think fuel pressure is okay. I am thinking that the problem is an intermittent connection from the Front O2 sensor, or the sensor itself, because the signal suddenly drops, then the Rear O2 sensor shows a mirror image reaction. My assumption is that the loss of voltage from the Front O2 sensor signals an increase in Fuel Trim, then the excessive fuel causes the voltage of the Rear O2 sensor to rise. If it was a vacuum leak, or low fuel pressure, I would think that the Front and Rear O2 sensor graphs would be similar, not mirror images.

Opinions? Comments?

Here are the three graphs:
SHORT TERM FUEL TRIM
(IMG:http://farm4.static.flickr.com/3114/2890414598_6a24c8ea8d.jpg)
FRONT O2 SENSOR
(IMG:http://farm4.static.flickr.com/3092/2889580117_fbe1c8abce.jpg)
REAR O2 SENSOR
(IMG:http://farm4.static.flickr.com/3200/2890414578_5a43acaaf2.jpg)

Another related question. My 1999 Diamante has the federal emissions setup, so there are only the two o2 sensors, one at the front and one at the back of the catalytic converter. I wanted to try reseating the plug to the front O2 sensor to test for an intermittent connection. The front O2 sensor wiring goes to a rubber grommet on the floorboard just above the O2 sensor. I tried pulling the rubber grommet loose, but it was firmly in place. Before I pry it harder I need to know if that is where the connection is, or do I have to open up the console and under-dash area to get to the connector?

they are HO2 sensors on this model. NOt O2 sensors

Yes, I know they are heated. I don't know how that distinction makes a difference in my question, but if it helps you to answer my question, assume I typed an "H" before O2 in each instance in my above messages.

Can anyone that has done the sensor replacement please let me know where the connector is? Thanks!

frankly, the most common reason that a diamante throws a lean code is due to the use of fuel. They are VERY sensitive to fuel. High alcohol content usually it what pops that code.

I dont actually think ive ever had a legitimate other reason for a lean code.

I had thought it may be bad fuel because the local stations are starting to sell gas with a small ethanol content, but if it was because of fuel, both the front and rear sensor would show a lean condition. But when the front sensor waveform goes lean, the rear one goes rich. Normally their waveforms track each other, with some delay and small variation.

I tried inspecting the connector and found the grommet hard to pull loose so I didn't go further until a got an answer to my second posting in this thread. However, moving the front sensor wiring has temporarily fixed my problem. I now get normal fuel trim and both front and rear sensor waveforms track normally. My guess is that after almost 10 years of heat, the wiring has an intermittent problem. Rather than take a chance it will come back, I ordered a new OEM Denso sensor ($60 instead of $300 at the dealer) and will change it.