1998 Nissan Maxima P0155 and P0325 Fault Codes: Causes, Diagnosis, and Repair Steps
My 1998 Nissan Maxima is showing both OBD-II diagnostic codes P0155 (O2 Sensor Heater Circuit Malfunction) and P0325 (Knock Sensor Circuit Failure). The vehicle runs normally with no noticeable idle issues or performance problems. I'm wondering how urgent it is to replace both components, and whether one fault could be causing the other. Based on my research, both codes indicate unexpected voltage readings in their respective circuits. P0325 relates to the knock sensor circuit (a single-wire sensor), while P0155 points to a heater circuit failure in the oxygen sensor — not the sensor itself. Since both sensors are typically located underneath the engine and share common wiring harnesses, it's possible they're affected by the same physical issues, such as wire abrasion or damage from rubbing against metal components. Recommended steps: 1. Visually inspect the wiring looms near the knock and O2 sensors to check for fraying, kinks, or points of contact with engine parts. 2. Test the knock sensor: With the key on but engine off, measure 5V at the connector. If absent, there's likely an open circuit. Reconnect and backprobe — a healthy sensor should show around 2.5V under these conditions. No voltage suggests a faulty knock sensor; persistent 5V indicates a possible issue with the sensor or its connector. 3. For the Bank 2 O2 sensor heater circuit: With the key on, engine off, check for 12V at the light green/blue wire (heater circuit). If present, it's easier to replace the sensor than attempt further troubleshooting of the heater circuit — especially for non-technical users. I'm planning to perform these checks this evening and will update accordingly.
P0325 typically does not trigger the check engine light, but P0155 should. Both codes point to abnormal voltage in their circuits. The knock sensor (a single-wire component) and the O2 sensor heater circuit are often located beneath the engine and may share a common wiring harness. Physical damage — such as rubbing against metal parts — could affect both sensors simultaneously. Step-by-step inspection: - Locate the Bank 1 knock sensor, unplug it, and with the key on but engine off, check for 5V at the connector. If absent, there's likely an open circuit. Reconnect and backprobe: a healthy sensor should show about 2.5V under these conditions. No voltage suggests a bad knock sensor; if you still read 5V, suspect either the sensor or its connector. - For Bank 2 O2 sensor heater circuit: With key on, engine off, check for 12V at the light green/blue wire (heater circuit). If present, replacing the sensor is simpler than diagnosing a heater circuit fault — especially for DIYers.