1999 Mercury Villager P0325 and P0135 Fault: O2 Sensor Replacement and Knock Sensor Short Diagnosis
I recently encountered both P0325 (Knock Sensor Circuit Malfunction) and P0135 (Oxygen Sensor Heater Circuit Malfunction) on my 1999 Mercury Villager. After replacing the oxygen sensor, I discovered a short near the Powertrain Control Module (PCM). Fixing this electrical issue cleared the P0135 code. However, before investing in a new knock sensor—which costs around $350 for parts and $500 in labor—I want to know what diagnostic steps I should take first to confirm whether there's another short or wiring issue affecting the knock sensor circuit.
It's always a good idea to test the electrical circuit before replacing expensive components like the knock sensor. The knock sensor on this engine model is known to fail, but when it does, it typically only stores a diagnostic code and doesn't trigger the check engine light. Its function is to detect engine knocking or pinging and send a signal to the ECU, which then adjusts ignition timing to prevent damage. I've seen many owners ignore knock sensor codes due to high repair costs—some simply let them go. That's their choice, but it's important to verify the issue first with proper diagnostics. Good luck with your troubleshooting.