Possible causes

– Air fuel ratio (A/F) Harness or connectors is open or shorted
– Air fuel ratio (A/F) sensor 1 Bank 1
– Air fuel ratio (A/F) sensor 1 heater Bank 1
– Fuel pressure
– Fuel injector
– Intake air leaks
– Exhaust Gas leaks
– PVC
– Air Flow Meter

When is the code detected?

The response (from RICH to LEAN) of the A/F signal computed by ECM from A/F sensor 1 signal takes more than the specified time.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Possible rough engine idle

P1278 Description

The Air Fuel Ratio (A/F) sensor is a planar dual-cell limit current sensor. The sensor element of the A/F sensor is the combination of a Nernst concentration cell (sensor cell) with an oxygen-pump cell, which transports ions. It has a heater in the element.

The sensor is capable of precise measurement