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.

2006 Mazda MX-5 Miata BaseFACTORY OEM SERVICE PROCEDURE

DTC P0138: Notes

DETECTION CONDITION AND POSSIBLE CAUSE (DTC P0138)

DTC P0138

Rear HO2S circuit high input

DETECTION CONDITION

The PCM monitors input voltage from rear HO2S. If the input voltage from the rear HO2S sensor is above 1.2 V for 0.8 s , the PCM determines that circuit input is high.

Diagnostic support note

This is a continuous monitor (HO2S).
The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
FREEZE FRAME DATA is available.
The DTC is stored in the PCM memory.

POSSIBLE CAUSE

Rear HO2S malfunction
Short to power supply in wiring harness between rear HO2S terminal A and PCM terminal 2Q
Rear HO2S or PCM terminal shorted
PCM malfunction

Courtesy of MAZDA MOTORS CORP.

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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.