Advanced Engine Performance Specialist Flashcards

A wide-band (air/fuel ratio) oxygen sensor differs from a conventional narrow-band sensor primarily because it can:

Answer: Measure the exact air/fuel ratio across a wide range

Wide-band sensors use a pump cell to report the precise AFR across a broad range rather than just switching at stoichiometry.

A vehicle shows high long-term fuel trim (e.g., +25%) at idle but normal trim at cruise. The MOST likely cause is:

Answer: A vacuum leak

A vacuum leak has the greatest proportional effect at idle when airflow is low, driving fuel trims positive.

When diagnosing a P0301 (cylinder 1 misfire), the technician should FIRST:

Answer: Verify the misfire follows the cylinder by swapping coil/injector

Swapping components to a different cylinder shows whether the fault moves, isolating coil/injector versus a mechanical issue.

A relative compression test using a current clamp on the starter is useful because it:

Answer: Identifies a weak cylinder without removing spark plugs

Cranking current variation reveals a low-compression cylinder quickly without disassembly.

A MAF sensor reading lower than expected grams-per-second at wide-open throttle will typically cause:

Answer: A lean condition and possible hesitation

An underreporting MAF tells the ECM less air is entering, so it commands less fuel, creating a lean condition.

Catalytic converter efficiency is best evaluated by comparing:

Answer: Upstream and downstream O2 sensor activity

A good converter stores oxygen, so the downstream sensor stays relatively steady compared to the switching upstream sensor.

A P0420 code most directly indicates:

Answer: Catalyst system efficiency below threshold (bank 1)

P0420 reports that bank 1 catalyst efficiency has dropped below the calibrated threshold.