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Old 03-30-2010, 06:22 PM
pzaremba's Avatar
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Originally Posted by tech22 View Post
You refer to both banks but there is only one bank.
2000 Nissan Sentra GXE with the QG18DE engine. 102,000 or so miles.

I have four O2 sensors, two per bank, front and rear (before/after the catalytic converter).

As I understand it from the service manual, the outer two cylinders are bank 1 and the inner two are bank 2.

Here's my story:

About a year ago, I was getting an intermittent P0171 code (Bank 1 System Too Lean), which would clear and re-appear. It finally stayed on. Seems to come on as I'm accelerating on the highway through about 2,500 RPM and 40-50 MPH.

It reliably re-appears after I clear the codes. Last week, the engine stated running very rough, and I was now getting codes

P0134, P0154 (Bank 1/2 front O2 "high voltage").

The PCM, through the OBD-II port, was reporting that both sensors were off-scale high, reading a steady +1.275 V. Direct voltage measurements using probes at the PCM terminals themselves were showing slowly fluctuating NEGATIVE voltages. The hotter the engine was, the more negative the voltage from the sensor. And it was a very slow change. Totally abnormal, it seemed. The sensors should be putting out between 0-1 V. In any case, the measurements I was taking off of the sensors didn't match with what the PCM was reporting.

So, I didn't know whether I had a ground problem, a power short, bad sensors, or a bad PCM.

I did a continuity check of the harness according to section EC, page 256. Harness OK.

I did the diagnostics for the O2 heater circuits as well, with normal OK results.

After consulting with a wise and seasoned electrical engineer (my father-in-law), I ended up rigging a test harness using my laboratory power supply and a resistor divider network to feed a known voltage into the PCM on pins 62 and 94 (the bank 1/2 front O2 sensor signal pins), and see what came out the OBD-II port.

Turns out the PCM was reporting the exactly correct voltages to within 1 mV, which was a huge relief. My theory is that the negative voltages from the sensors were confusing the analog-to-digital conversion in the PCM and it was freaking out and interpreting the negative voltage as an off-scale high value.

So, having eliminated the PCM and the harness, the sensors were the only things left as suspect. I replaced both front O2 sensors with Bosch exact-fit OEM replacement parts, and reset the PCM by leaving the battery disconnected, then performing the "P100" procedure by disconnecting/reconnecting the MAF.

The car ran like crap, and I was doing circuits around the house within a 5-block radius or so in case I got stranded. After a few miles, it started running very well, so I ventured out onto the highway and did a few high-speed runs back and forth. The car was running GREAT! Smoothest idle I've had in a long time, good acceleration, good power, and the sensors were putting out a nice 0-1 V oscillating signal.

I thought I had everything fixed.

Today, on the way to work, I got the dreaded P0171 DTC pending-only, meaning no SES light. On the way home, it became a full-fledged DTC and I dumped out the freeze frame as follows:

Looks like the long-term fuel trim is way too high. Perhaps it is the MAF? Then why only a P0171 and not a P0174 as well?

My guess is that the O2 sensors finally went out on me, and are unrelated to the original P0171 problem. I spent the weekend over-analyzing O2 sensors that finally went bad while not really addressing the original problem.

Should I be looking for an intake or exhaust leak, does the PCM need to be "reprogrammed", or is this really the sign of a defective MAF sensor?

I don't understand what tech22 says about "only one bank". Everything I'm reading and seeing indicate that I have two banks.

The Sentra is running better than it has been since I can remember, but I'm still unable to get my emissions tested until I fix this one.

Let me add that this site has been a very, very useful thing to me and I'm extremely grateful for it and all of the people who contribute!

Last edited by pzaremba; 03-31-2010 at 12:42 AM.
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