Practice Test #5 1 Flashcards

A technician observes Long Term Fuel Trim (LTFT) values of +22% at idle that drop to approximately +5% at 2,500 RPM. Which condition BEST explains this pattern?

Answer: A vacuum leak introducing unmetered air primarily at idle

Vacuum leaks allow unmetered air to enter the intake manifold. At idle the leak represents a large percentage of total airflow, so the PCM compensates heavily with positive fuel trim. At higher RPM, commanded airflow dwarfs the leak volume and fuel trim normalizes. A bad MAF would show elevated trim across all RPM ranges.

A downstream (post-catalyst) oxygen sensor displays the same rapid, high-amplitude switching pattern as the upstream sensor. What does this waveform indicate?

Answer: The catalytic converter has low conversion efficiency

A healthy catalyst stores excess oxygen and releases it as needed, causing the downstream O2 sensor to switch slowly and with low amplitude. When the downstream sensor mimics the upstream sensor's rapid switching, the catalyst is no longer buffering oxygen, indicating it has lost its conversion efficiency.

A vehicle equipped with ABS consistently pulls to the right during a hard stop while ABS is actively cycling. Brake fluid level and pad thickness are within spec on all corners. Which component should the technician inspect FIRST?

Answer: The right-front wheel speed sensor and tone ring

ABS individually modulates brake pressure at each wheel based on wheel speed sensor data. If the right-front sensor is providing an incorrect signal, the ABS module may prematurely release or apply pressure on that corner, creating a pull toward the side with the faulty sensor input.

An automatic transmission slips specifically during the 2nd-to-3rd gear upshift under heavy throttle. Fluid level, color, and smell are all normal. No other gear changes are affected. Which component is MOST likely at fault?

Answer: The 2-3 shift solenoid or its wiring circuit

Each shift solenoid in an automatic transmission controls a specific gear change. A faulty 2-3 shift solenoid or an open/short in its circuit causes the clutch pack or band for 3rd gear to engage incompletely, resulting in slippage only during that transition. Other gear changes use different solenoids and are unaffected.

A smoke test reveals no leaks in the EVAP system. The purge solenoid opens on command and the vent solenoid closes on command, yet the EVAP monitor continues to report incomplete. Which component is MOST likely preventing monitor completion?

Answer: A faulty fuel tank pressure (FTP) sensor

The EVAP monitor seals the system (closes the vent, opens purge) and then uses the fuel tank pressure sensor to confirm vacuum or pressure builds within the system. If the FTP sensor is faulty or out of calibration, the PCM never receives confirmation that the system sealed, so the monitor aborts even though the hardware is functioning correctly.

A dial indicator measurement shows 0.009 inch of lateral runout on a front brake rotor. The manufacturer's maximum specification is 0.003 inch. Which customer symptom is MOST consistent with this finding?

Answer: A pulsating brake pedal and steering wheel shimmy during normal braking

Excessive lateral runout causes the rotor to wobble side to side as it rotates. Each revolution pushes the brake pads outward and then allows them to relax, creating a pressure pulse in the hydraulic circuit that the driver feels as pedal pulsation. The resulting force variation can also transmit through the steering linkage as a shimmy.