P0138 fault code: what it means

P0138: Bank 1 downstream O2 sensor voltage stays high.

Engine and gearbox (powertrain) · Minor

Usual causes, most likely first

1. failed rear O2 sensor
2. real rich mixture (leaking injector, high fuel pressure)
3. signal wire shorted to voltage

What the repair usually costs

• Air-fuel ratio / front O2 sensor replace (upstream)NZ$270–NZ$750 · A$260–A$730
• O2 sensor replace (downstream / after cat)NZ$180–NZ$480 · A$180–A$460
• High-pressure fuel pump (GDI petrol or common-rail diesel)NZ$1,140–NZ$4,280 · A$1,060–A$3,800

Not sure what P0138 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.