What does it mean?
P1273 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Air Fuel Sensor (Bank 1 Sensor 1) Circuit Lean Shift .
Symptoms
- The MIL is on, often with lean fuel trim codes stored alongside
- Fuel trim values well above normal at idle and cruise
- Hesitation, flat spots and a lack of power under acceleration
- Rough or unstable idle, occasional stalling
- Exhaust ticking or hissing when a leak is the cause
- Increased fuel consumption despite the lean condition
Likely causes
- Exhaust leak ahead of the sensor drawing fresh air into the stream
- Vacuum leak at an intake gasket, hose or the brake servo line
- Low fuel pressure from a tired pump, clogged filter or a leaking regulator
- Restricted or dirty injectors delivering less fuel than commanded
- Contaminated or aged air flow sensor under reporting incoming air
- Sensor itself drifting lean after high mileage or silicone contamination
- Wiring resistance in the sensor signal circuit shifting the reading
How to fix it
- Read short and long term fuel trims at idle and at raised rpm to judge how far the correction has moved
- Listen and inspect for an exhaust leak between the manifold and the sensor, including cracked manifolds and loose flange bolts
- Perform a smoke test or careful visual check of the intake for vacuum leaks
- Measure fuel pressure under load and compare it with specification
- Compare the air flow sensor reading with the expected value for engine size and rpm
- Inspect the sensor wiring and connector for chafing, corrosion and voltage drop
- Correct the leak or fuelling fault first, then replace the air fuel ratio sensor only if the lean shift remains with everything else proven good