Code status
Needs verification
Not yet verified against SAE J2012_202509 / J2012DA. Treat unverified rows as a lookup aid, not repair authority.
- System
- Fuel / Air
- Scope
- iso-sae-controlled-p0
- Standard
- SAE J2012 / ISO 15031-6
- Source
- legacy-import
Diagnostic boundary
A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.
Do not treat P0138 as a catalyst diagnosis by itself; verify the sensor circuit and engine mixture first.
Useful checks before replacing parts
- Compare downstream sensor voltage to upstream sensor activity and fuel trims.
- Inspect the downstream sensor harness for melting, short-to-voltage, water intrusion, or connector damage.
- Check for rich-running conditions or catalyst-monitor faults that can influence downstream readings.
Related OBD-II codes
These nearby fuel / air entries can help compare wording and fault area. They may share the same verification status as the source database.