ASE Practice Test #5 2

Question 1

A technician performs a relative cylinder compression test (cylinder contribution test) and finds cylinder 4 has the lowest contribution. What is the NEXT step to determine the root cause?

Accepted Answer

Perform a conventional compression test followed by a cylinder leakdown test on cylinder 4 to differentiate between compression loss (rings, valves) and ignition/fuel issues

Answer

The relative compression test identifies WHICH cylinder is weak, not WHY it is weak. The follow-up is a compression gauge test to quantify the actual cylinder pressure, and if low, a cylinder leakdown test to determine where the compression is leaking (past rings — heard at oil filler; past exhaust valve — heard at exhaust; past intake valve — heard at intake; past head gasket — seen as coolant bubbling). This systematic approach identifies the specific mechanical fault before disassembly.

Question 2

What does a KOEO (Key On Engine Off) test reveal that a KOER (Key On Engine Running) test does not?

Accepted Answer

KOEO tests check electrical circuit integrity and hard faults stored in memory without the influence of running engine conditions; KOER tests additional sensor and actuator performance under operating conditions

Answer

KOEO (Key On Engine Off) self-tests check the electrical circuits connected to sensors and actuators — looking for opens, shorts, and out-of-range static values. Since the engine isn't running, transient faults from operating conditions don't interfere. Hard faults that exist at all times show up clearly. KOER (Key On Engine Running) tests check sensor and actuator performance under actual operating conditions, including dynamic sensor responses and system interactions. Both tests are part of complete OBD-I and some OBD-II manufacturer-specific diagnostic procedures.

Question 3

A technician measures a thermistor (NTC type) sensor resistance. As the temperature increases, what happens to the resistance reading?

Accepted Answer

Resistance decreases as temperature increases — NTC stands for Negative Temperature Coefficient

Answer

NTC (Negative Temperature Coefficient) thermistors are used for coolant temperature sensors (CTS/ECT), intake air temperature sensors (IAT), and transmission temperature sensors. As temperature rises, electrical resistance through the NTC thermistor decreases. The ECM reads the resulting voltage drop across the sensor (in a voltage divider circuit) and converts it to temperature using a lookup table. A failed sensor stuck at high resistance causes the ECM to see a false cold signal, causing rich running and no cooling fan activation.

Question 4

What is the function of variable displacement (cylinder deactivation) technology in a V6 or V8 engine?

Accepted Answer

To deactivate specific cylinders (closing their valves and stopping fuel injection) during light load conditions, reducing the effective engine displacement and significantly improving fuel economy

Answer

Cylinder deactivation (Active Fuel Management/GM, MDS/Chrysler, VTEC/Honda) shuts down half of the engine's cylinders during light load conditions (highway cruise, gentle acceleration). The system closes the intake and exhaust valves on deactivated cylinders and cuts fuel injection to them. The remaining active cylinders work harder to maintain vehicle speed, operating at a more efficient throttle position with less pumping loss. The transition is controlled by solenoids actuating special lifters that collapse, preventing valve actuation.

Question 5

A technician uses a scan tool and finds the O2 sensor upstream (pre-catalyst) is switching normally, but the downstream O2 sensor reads a steady 0.1V. What does a steady low voltage downstream O2 sensor indicate?

Accepted Answer

Permanently lean downstream exhaust — possible causes include an exhaust leak between the upstream sensor and converter, or a failed catalyst that is not processing the exhaust gases at all (letting lean exhaust pass through)

Answer

A downstream O2 sensor that reads a steady low voltage (approximately 0.1V, indicating lean) suggests very lean exhaust consistently reaching the downstream sensor. Possibilities include: a major exhaust leak between the upstream sensor and the converter (diluting with fresh air), a severely failed catalyst that has no conversion activity (allowing raw lean exhaust through), or a failed downstream O2 sensor stuck at a low value. Comparing with a vacuum leak diagnosis and exhaust inspection helps differentiate these causes.