2000 Mercury Villager O2 Sensor and Knock Sensor Codes: Diagnosis and Fixes

You should likely replace the post-catalytic converter O2 sensor (Bank 1). Also, inspect the wiring and connections for both the O2 sensor and knock sensor. Check the knock sensor harness as well—wiring issues are common in this model. Good luck and please update us on your progress.

Thanks for the reply, Mohawkmtrs. I have a few follow-up questions: Where is the knock sensor wiring harness located on my 2000 Mercury Villager? Is there a specific procedure to test the knock sensor? Is Bank 1 (the O2 sensor) located on the side of the engine closest to the firewall, given that it's a transverse V6? Should I replace both O2 sensors if one fails? I'm thinking that if one goes bad, others might fail soon—like replacing both headlights when one burns out.

Inspect the wiring thoroughly for the O2 sensor. I don’t remember the exact location without seeing it, but near the front engine mount—the harness often breaks or corrodes right where it connects to the mounting bracket. If the wiring is intact and the connection is good, a bad O2 sensor is likely the issue. The PO325 knock sensor code should not trigger the check engine light on its own. So if you fix the O2 sensor, the knock sensor code may return—but the light should stay off.

Thanks for all the replies. I tried resetting the codes and have been monitoring the system. So far, no recurrence—light has not come back on.

The codes returned, so I replaced the post-catalytic converter O2 sensor (Bank 1 Sensor 2). I had some difficulty disconnecting the harness as it was buried behind the alternator, but otherwise the process went smoothly. The PO138 code is now resolved. However, the knock sensor code (PO325) remains. The car runs fine, so I’m not planning to replace the knock sensor—it’s a complex and labor-intensive job.

Now I have two new codes: PO135 (O2 Sensor Heater Circuit Malfunction - Bank 1 Sensor 1) and PO141 (O2 Sensor Heater Circuit Malfunction - Bank 1 Sensor 2). I replaced the post-cat O2 sensor last week. I installed a Bosch universal O2 sensor, and I’m confident I didn’t miswire the connector. Could it be that the pre-catalytic converter O2 sensor is now failing and causing both new heater circuit codes?

NEVER use a universal O2 sensor—most have incorrect heater resistance, which will trigger the heater circuit fault. Always verify the heater resistance matches OEM specifications. Also, inspect the wiring carefully—O2 harnesses are prone to failure in this vehicle model.

Thanks for the advice. I didn’t actually install a universal sensor—I only asked for an after-cat O2 sensor at the parts counter. Lesson learned: I should have verified the part type. UPDATE: I discovered that fuse #7.5 (7.5-amp) for the O2 sensors had blown. After replacing it, the check engine light returned after a few days—now showing PO135 and PO141 (heater circuit faults), but not the original codes. I didn’t get a chance to verify if the fuse blew again, but I suspect that’s what happened. Could the Bosch universal sensor have caused the fuse to blow? I don’t believe there's a wiring issue because the original codes only pointed to the post-cat O2 sensor. Now I'm seeing faults for both sensors. I plan to return the universal sensor and install an OEM-style one to ensure full system reliability.

Replace the Bosch O2 sensor with an original equipment manufacturer (OEM) unit, recheck all wiring and connectors—don’t forget the 7.5-amp fuse. If you didn’t have heater circuit codes before installing the Bosch sensor, it’s highly likely that switching to a proper OEM sensor will resolve the issue (assuming the harness is in good condition). Please let us know how your fix turns out.

Problem solved—I installed an OEM-style O2 sensor and replaced the 7.5-amp fuse. The check engine light has been off since, and all codes are cleared. Thanks to everyone for your helpful advice!