Mazda 3 2L Model P0032 and P0038 O2 Sensor Heater Circuit Errors After Header Installation

What brand of oxygen sensors are you using? Did the check engine light activate immediately after installing headers? Are there any visible exhaust leaks near the O2 sensor locations? Have you tested the wiring with a digital multimeter (DVOM)? Do you have access to an official wiring diagram for your Mazda 3 model?

The oxygen sensors I installed are genuine Mazda NTK parts. I'm not familiar with a DVOM, but I did use a multimeter to test the wiring. The exhaust was professionally inspected and found to have no leaks. I don’t currently have an official wiring diagram, though every time I replace a sensor, I inspect the wiring from the sensor to its connector — and it appears intact. The check engine light didn't come on right after installation. This is what happened: When the headers were installed and I started driving home, everything was fine for the first 20 minutes. Then, around that time, the CEL lit up. Upon investigation, I found that the original installers had not properly reconnected the oxygen sensor wiring looms — instead, they left them exposed and in contact with the headers, causing melting and short circuits. I replaced both sensors and their wiring harnesses the following week. The light remained on. Each time I cleared the code, it returned shortly after driving. I’ve now replaced the sensors twice more — but the issue persists. Is there a possibility that a fuse has blown? Does anyone have any idea what’s actually going wrong here? Thanks for your help!

The wiring must be damaged. This explains why the codes keep appearing. Even if you’ve tested voltage and signal, that doesn’t guarantee all wires are intact. You should now perform a full wire-by-wire inspection — not just for voltage or signal, but also for continuity and resistance. Look specifically for open circuits or broken connections. A damaged ground wire could be the culprit, even if it’s not immediately obvious.

The wiring that was initially damaged has been replaced three times now. That's exactly what I'm saying — after replacing both sensors and their harnesses, the codes still return. This shows that the root cause may lie beyond just the sensor or its direct wiring.

It’s highly likely that a fuse or fusible link responsible for powering the oxygen sensor heaters has failed — possibly due to the initial short circuit from the faulty header installation. The PCM may have been affected by the electrical surge. A DVOM stands for Digital Voltage Ohm Meter — you’ll need one and an official wiring diagram to properly diagnose this issue. Otherwise, you might end up needing a new PCM, which is expensive and not typically covered under warranty if the damage was caused by improper installation.

I'll try to locate a reliable wiring diagram for my Mazda 3. Hopefully, it's a blown fuse — I really don’t want to spend money on a new PCM! Does anyone else have any other ideas? Thanks again for the help!

Okay — also check the O2 sensor ground wires. It's possible that during the header installation, one of the grounding connections was accidentally disconnected (some sensors rely on a small bolted wire connected to engine or body metal). For example, if a single ground wire is missing or damaged, you may not notice it immediately — but the sensor will lose its signal because it needs a proper ground path. When I refer to O2 ground wires, I’m not necessarily talking about the wire directly from the sensor. There are multiple grounding points on the engine block or engine compartment that supply ground signals for various sensors — one of these could be responsible for the 02 sensor readings. Please inspect all accessible ground connections in the engine bay and ensure they’re securely attached and undamaged. Please keep me updated!

UPDATE: Today I checked the fuses related to the oxygen sensors — and now I feel incredibly foolish for not checking them earlier! Both fuses are completely missing. I believe the exhaust shop removed them, thinking it would fix the O2 light. I have no idea why anyone else would do this. Thanks to everyone who helped point me in the right direction!

Thanks for sharing your update — many users never return with results. I'm glad you found a solution and it didn’t require replacing the PCM.

That’s hilarious! Probably the wires melted before they returned the car, and the shop just forgot to replace them — or worse, never ordered new ones!

Could someone please specify where the O2 sensor fuses are located in a Mazda 3 2L model? I’d like to locate them before replacing any parts.