Mazda Miata CEL Fix: O2 Sensor Heater Circuit Code Diagnosed on a Miata

Today’s customer brought their car in because the check engine light was on. This, on its own, wasn’t completely out of the ordinary, but other than a lit-up check engine light, there were no other driveability concerns of note. Before bringing the car to the shop, the customer had a local autoparts store scan his 2005 Mazda Miata for fault codes, and they found one fault code related to the O2 sensor: an O2 sensor heater circuit 2 code. Other than the check engine light being on in the car’s dashboard, no other drivability concerns were present.

After a quick test drive to confirm there were no drivability concerns, I scanned this Mazda for fault codes and monitored live engine data to identify the fault. The vehicle only had 82000 miles on the odometer.

Using my full-system scanner, I scanned for fault codes and monitored live engine data to observe the O2 sensor 2, heater control, and voltage.

The sensor heater circuit showed 0 volts as the engine warmed up, but reached operating temperature.

The vehicle was raised, and I began the repair by inspecting the exhaust system, the sensor, and its wiring. No noticeable damage to the sensor or to the wiring was present. Using ProDemand, I pulled up the vehicle’s wiring diagram to find the location of this Mazda’s O2 wiring harness and its connection points, and how it communicated with the ECM.

After checking continuity between the ECM and O2 sensor connections, I confirmed that the wiring was undamaged and that there was no poor connection between the two components. From there, I decided to check resistance through the connections for ground, power, on the O2 sensor, and the heater control circuit. No connection was above .06 ohms. The only thing to check was the sensor component itself. I removed the O2 sensor from the exhaust to inspect the sensor end for any damage. No noticeable damage was found.

After a thorough inspection, all tests pointed to a faulty O2 sensor.…