P0135 O2 Sensor Heater Circuit Fault - 2001 Mitsubishi Eclipse GS 4-Cylinder Diagnosis and DIY Repair Guide

The P0135 code indicates a heater circuit malfunction in the upstream oxygen sensor (O2) located on Bank 1 of your 2001 Mitsubishi Eclipse. This sensor is typically situated near the exhaust manifold. Before replacing it, perform these checks: - Verify that all relevant fuses are intact. - Inspect the wiring harness and connector for damage or corrosion. If no issues are found, replacement of the O2 sensor is likely needed. For best results, purchase a genuine OEM part from the dealership rather than aftermarket brands — this ensures compatibility and proper function. To replace: 1. Disconnect the negative battery terminal. 2. Locate the upstream O2 sensor (it's mounted near the exhaust manifold). 3. Unplug the electrical connector. 4. Use an open-end wrench (typically 22mm) to loosen and remove the sensor from the exhaust pipe — avoid damaging any wiring. 5. Apply a small amount of anti-seize compound (included with most new sensors) before re-installing the replacement. 6. Tighten securely, reconnect the harness, and restore battery power. 7. Start the engine; if the check engine light remains on, visit AutoZone or O'Reilly's to clear the code using their OBD2 scanner. Note: Replacement parts can be expensive — typically $80–$150 at dealerships. However, you may find compatible upstream O2 sensors for around $25–$50 on eBay. When searching, ensure the listing specifies 'upstream' and confirms compatibility with your 2000–2001 Eclipse model. Since Eclipse vehicles have two separate banks of O2 sensors (bank 1 and bank 2), only the upstream sensor should be replaced to avoid code reoccurrence. If you're unsure or uncomfortable, consider having a mechanically experienced friend assist — it's safer and more reliable.

Thank you so much! I'll try this step-by-step and update you on my progress.

Hi, I saw your post. Here's a quick follow-up: While the exact location of the O2 sensor in 2000–2001 Eclipse models may vary slightly, the upstream O2 sensor (Bank 1) is generally found near the exhaust manifold, running down from the header pipe toward the bottom of the vehicle. Step-by-step replacement: - Turn off the engine and disconnect the negative battery terminal. - Follow the wiring to locate the sensor’s connector. - Unplug the electrical connection. - Use a 22mm open-end wrench to loosen and remove the sensor from the exhaust bung — be careful not to damage any wires. Install the new sensor: - Apply anti-seize (usually provided with the part) to the threads. - Screw it back into place and tighten securely. - Reconnect the wiring harness. - Reattach the negative battery terminal. - Start the car. If the 'Service Engine Soon' light stays on, visit AutoZone or O'Reilly's to clear the code using their OBD2 scanner. Part cost: Genuine OEM sensors range from $80–$150 at dealerships. For savings, check eBay — search for 'upstream O2 sensor 2000-2001 Mitsubishi Eclipse'. Look for listings that confirm compatibility and label the sensor as 'upstream'. Remember, Eclipse models have two separate banks of oxygen sensors (bank 1 and bank 2), so only replace the upstream one to prevent code recurrence. This repair is manageable with basic tools — just ensure you follow each step carefully.

THANK YOU, THANK YOU, THANK YOU!!! A friend recently replaced the sensor and my 'Service Engine Soon' light has completely disappeared. I truly appreciate your help everyone! This was exactly what I needed to know.

Great to hear it's fixed! Just a quick reminder — if you used an aftermarket O2 sensor, the P0135 code might reappear. Always verify compatibility and ensure the part is labeled as 'upstream' for your model year and trim.