ASE Practice Test #3 3 Flashcards

A technician checks a vehicle with a complaint of rough idle. The scan tool shows STFT at +18% at idle and +2% at cruise. What does this pattern indicate?

Answer: A lean condition specifically at idle — the ECM is adding 18% more fuel to compensate for less fuel or more air than the base map expects at idle only; the cruise correction being normal rules out a global fuel system issue

Fuel trim patterns provide diagnostic information about where the fault occurs. A large positive STFT at idle (+18%) that normalizes at cruise (+2%) means extra unmetered air enters only at idle — the throttle opening is the smallest at idle, so even a small vacuum leak has a large lean effect at idle. At cruise (larger throttle opening), the same leak's effect is diluted. This pattern points to idle-speed vacuum leaks: EGR valve, intake manifold gasket at idle, throttle body gasket, or brake booster vacuum line.

What type of circuit fault causes the voltage in a parallel circuit branch to be higher than normal when that branch's component is faulty?

Answer: An open circuit in that branch — voltage rises to source voltage because no current flows and no voltage is dropped across the failed component

In a parallel circuit, each branch operates at source voltage when functioning normally. If one branch develops an open circuit (broken wire, failed component with open winding), current stops flowing in that branch. Without current flow, there is no voltage drop across the load in that branch — all source voltage appears across the open point. This is called an "open-circuit" fault. The component does not operate, and voltage measured across the open will read source voltage (Kirchhoff's voltage law).

A vehicle is brought in for an oil change. The customer mentions a grinding noise from the front end when turning. Which likely cause should the service advisor note for further diagnosis?

Answer: Worn front wheel bearing or failing CV joint — both can produce grinding or clicking noises during turns and require prompt inspection

A grinding noise during turns can indicate a failing front wheel bearing (grinding often load-dependent and speed-related, varying with turn direction as weight shifts) or a worn CV joint (typically produces clicking under load during tight turns). Both are safety-critical components — a severely worn wheel bearing can cause wheel separation. The service advisor should document this complaint and recommend a front end inspection before the noise progresses to a more expensive or dangerous failure.

What is the main advantage of electronic throttle control (drive-by-wire) over a conventional mechanical throttle cable?

Answer: It enables precise ECM control of throttle position for traction control, stability control, cruise control, idle speed, and emissions management without mechanical linkage

Electronic throttle control (ETC or "drive-by-wire") replaces the mechanical cable between the accelerator pedal and throttle body with an electronic system. The pedal has a position sensor; the throttle body has an electric motor. The ECM controls throttle opening. This enables traction control to close the throttle independently of the driver, stability control to reduce torque, precise idle speed management without a separate idle air control valve, and seamless cruise control operation. It also enables features like torque-based engine management and active safety systems.

When bleeding brakes on a vehicle with ABS, why might a scan tool be required?

Answer: Some ABS systems trap air in the ABS hydraulic control unit (HCU) valves — the scan tool cycles the ABS solenoids to open each valve and allow air to be purged from the HCU passages

Standard brake bleeding flushes fluid through the wheel cylinders and calipers but does not necessarily clear air from the ABS hydraulic control unit (HCU). The HCU contains multiple solenoid valves for each wheel circuit. After an ABS activation event (which can introduce air) or after HCU replacement, air can be trapped in individual valve passages. A factory or compatible scan tool can perform an automated ABS bleed routine, cycling each solenoid valve open in sequence to allow trapped air to pass through to the caliper and be bled out normally.

What is the purpose of the secondary air injection (AIR) system in the emissions control system?

Answer: To inject fresh air into the exhaust stream during cold start to promote combustion of unburned HC and CO, helping the catalytic converter reach operating temperature faster

The catalytic converter requires temperatures of approximately 300°C (572°F) to efficiently convert HC, CO, and NOx. During cold start, the converter is cold and ineffective. The AIR system injects fresh air directly into the exhaust manifold (upstream of the converter). Unburned HC and CO react with this oxygen and combust in the hot exhaust stream. This exothermic reaction heats the exhaust rapidly, bringing the converter to light-off temperature within 30-60 seconds instead of several minutes, dramatically reducing cold-start emissions.