Q: What would cause brand new O2 sensors to give codes 172, 173, 176, and 177, rich and lean conditions, both sides at the same time?

asked by on August 26, 2016

My car has an auto trans, and 4.0L V-6 engine. I have changed the MAF sensor and BOTH O2 sensors as recommended by yourmechanic.com. That recovered the acceleration and seemed fine driving around town on a Friday. Monday I drove the truck to work on the freeway, 48 miles each way. In the morning at 3am it was mostly fine though the check engine light came on 6 times. going up grades it loses speed, then seem like it down shifted about a 400 rpm rise and recovered the acceleration. In the afternoon, all of the former issues came back. Codes were 172 and 176 (O2 sensor lean conditions both sides) and 173 and 177 (O2 sensor rich conditions both sides). I cleared the codes and it seems to drive normally in town, but the check engine light came on 3 times during a 13 mile drive. (same codes) Stopping and restarting the engine will turn off the check engine light. What will cause new O2 sensors to report lean and rich and cause a check engine light and acceleration issues?

My car has 95700 miles.
My car has an automatic transmission.

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When it comes to across-the-board codes being set such as this, it is usually a result of something "upstream" creating the condition to set all these codes. For instance, it is rare for both O2 sensors to fail at exactly the same time; this is likely an indication of something else going on. The most common cause of such symptoms is a vacuum leak.

Vacuum leaks will can come and go depending on RPM and manifold pressure. Your truck will struggle to find an equilibrium and will make extreme adjustments attempting to find the correct fuel mixture. A common place for a vacuum leak is in the tube that connects the air filter box to the throttle body. These tubes are flexible and can seal and open with motor movement and air flow into the motor. Look for other vacuum leaks as well.

Another possibility is fuel-contaminated oil. The positive crankcase ventilation (PCV) system is designed to draw in the fumes in the crank case for emissions reasons. If there is fuel leaking into the oil, or the oil has not been changed on a regular basis, trouble codes can be set as the fumes in the crank case are cleared out and build back up.

If you are using a K&N filter charged with oil, this can contaminate your mass airflow (MAF) sensor and cause all of these problems. I have seen many people chase this problem only to find that the oil on the K&N filter is the problem. Also, some replacement MAF sensors are better than others. It is possible you have a bad one. In addition, check the wiring harness to the MAF sensor, and to both O2 sensors. The wiring harness can be host to a wide variety of issues.

Last, the Powertrain Control Module (PCM) could be failing. Testing this is an extensive process. You have to directly monitor the MAF and 02 sensor signals separate from the PCM while comparing them to the data that the PCM is displaying on a scanner. If they don’t agree with each other, you likely have a PCM failure.

Check for leaking injectors as well. Leaking injectors will allow excessive fuel into the intake and the PCM will continually over compensate. The same thing can happen with the fuel pressure regulator.

So as you can see, there is a lot that can cause these codes. I recommend booking an appointment with one of our technicians to help you figure this out. A Check Engine Light inspection is a great place to start. Make sure to include as many details about your problem as possible when you book your appointment.

Good luck!

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