P0138moderate severityPowertrain
O2 Sensor Circuit High Voltage Bank 1 Sensor 2
on 2004 Mazda MX-5 Miata 1.8L I4 · RWD
What this code means
The downstream O2 sensor on Bank 1 is reading high voltage consistently.
Symptoms
- Check Engine Light
- Failed emissions
- May indicate catalyst failure
Common causes
- Faulty sensor
- Catalytic converter failure
- Rich running condition
Diagnostic steps
- Test sensor voltage
- Check catalyst efficiency
- Monitor upstream vs downstream O2 patterns
Repair notes
Replace sensor. If catalyst failed, replace catalytic converter.
Other Powertrain codes on this vehicle
P0001
Fuel Volume Regulator Control Circuit/Open
P0002
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P0003
Fuel Volume Regulator Control Circuit Low
P0004
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P0010
Intake Camshaft Position Actuator Circuit/Open Bank 1
P0011
Intake Camshaft Position Timing Over-Advanced Bank 1
P0012
Intake Camshaft Position Timing Over-Retarded Bank 1
P0013
Exhaust Camshaft Position Actuator Circuit/Open Bank 1
P0014
Exhaust Camshaft Position Timing Over-Advanced Bank 1
P0015
Exhaust Camshaft Position Timing Over-Retarded Bank 1