Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Heated Oxygen Sensor (Ho2S): Millenia
1999 Mazda Protege ES, AutomaticSECTION Millenia
- Warm engine to normal operating temperature. Connect scan tool to Data Link Connector (DLC). DLC is located under left side of instrument panel, right of steering column. Using scan tool, access DIAGNOSTIC DATA LINK menu, and select PCM. Highlight and select PID/DATA MONITOR AND RECORD.
- Turn ignition on. Select HO2S PID to be checked. See HO2S PID IDENTIFICATION (MILLENIA) table. Voltage should be .5-1.0 volt under acceleration and 0-.5 volt under deceleration. If voltage is not as specified, check wiring harness and connector between PCM and HO2S. Also check intake manifold system, fuel system and self-diagnostic system. Repair as necessary. If no problems are found, replace HO2S.
- To test HO2S heating element on 2.5L, go to next step. To test HO2S heating element on 2.3L, allow sensor to cool to room temperature. Disconnect HO2S connector. Using ohmmeter, measure resistance between sensor terminals "B" and "C" for front sensor or terminals "C" and "D" for rear sensor. See Fig 1 . Resistance should be about 6 ohms with sensor temperature at 68°F (20°C). If resistance is not as specified, replace HO2S.
- To test HO2S heating element on 2.5L, allow sensor to cool to room temperature. Disconnect HO2S connector. Using ohmmeter, measure resistance between sensor terminals "C" and "D". See Fig 2 . Resistance should be about 6 ohms with sensor temperature at 68°F (20°C). If resistance is not as specified, replace HO2S.
HO2S PID IDENTIFICATION (MILLENIA)
| HO2S | PID | |
|---|---|---|
| 2.3L | ||
| Left Front Sensor | FHO2S L | |
| Left Rear Sensor | RHO2S L | |
| Right Front Sensor | RHO2S R | |
| Right Rear Sensor | FHO2S R | |
| 2.5L | ||
| Left Front Sensor | FHO2S L | |
| Middle Sensor | MHO2S | |
| Rear Sensor | RHO2S | |
| Right Front Sensor | FHO2S R | |
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.