NCT Case Analysis & Practical Application 3 — Questions and Answers
Question 1: A technician is diagnosing a fleet vehicle that stalls at idle but runs fine at higher RPMs. Fuel pressure tests are within specification. What system should be inspected next?
- Fuel injector spray pattern
- Idle air control (IAC) valve or throttle body (Correct answer)
- Ignition timing advance mechanism
- Exhaust gas recirculation (EGR) valve at wide-open throttle
Correct answer: Idle air control (IAC) valve or throttle body
A faulty IAC valve or dirty throttle body restricts the air bypassed around the throttle plate at idle, causing stalls despite normal fuel pressure.
Question 2: During a fleet vehicle inspection, the technician finds that the serpentine belt is cracked but not yet frayed. Company policy requires replacement only upon failure. What is the correct action?
- Follow company policy and leave the belt in service
- Replace the belt immediately and document the deviation from policy with justification (Correct answer)
- Spray belt dressing to temporarily prevent further cracking
- Notify the driver and allow them to decide
Correct answer: Replace the belt immediately and document the deviation from policy with justification
A cracked serpentine belt presents an imminent failure risk that could strand the vehicle or disable the charging and cooling systems; replacement is the technically correct safety action and should be documented.
Question 3: A fleet technician encounters a diesel truck that has excessive black smoke from the exhaust under load. What is the FIRST system to evaluate?
- Exhaust aftertreatment system (DPF)
- Fuel injection system and air intake restriction (Correct answer)
- Engine coolant temperature sensor
- Transmission shift quality and torque converter lock-up
Correct answer: Fuel injection system and air intake restriction
Black smoke under load in a diesel engine indicates an overly rich air-fuel mixture, most commonly caused by a restricted air intake or faulty fuel injection system delivering too much fuel.
Question 4: A fleet manager reviews telematics data and finds that one driver consistently shows hard braking events five times more often than peers on the same routes. What is the appropriate fleet management response?
- Immediately terminate the driver's employment
- Schedule a one-time vehicle inspection for brake wear
- Conduct a coaching session with the driver and monitor subsequent telematics data (Correct answer)
- Reassign the driver to a shorter route to reduce braking opportunities
Correct answer: Conduct a coaching session with the driver and monitor subsequent telematics data
Driver coaching using objective telematics data is the standard industry approach to correct unsafe driving behaviors before they result in accidents or excessive component wear.
Question 5: A fleet vehicle's OBDII scanner shows a P0420 code (Catalyst System Efficiency Below Threshold). Before condemning the catalytic converter, what should the technician verify?
- Tire pressure and wheel alignment
- Upstream and downstream O2 sensor operation and any exhaust leaks (Correct answer)
- Fuel tank capacity and fuel grade being used
- Transmission fluid condition and level
Correct answer: Upstream and downstream O2 sensor operation and any exhaust leaks
A faulty upstream or downstream O2 sensor, or an exhaust leak between the sensors, can falsely trigger P0420 before the catalytic converter itself is at fault.
Question 6: A Class 8 truck fleet experiences repeated premature failure of wheel bearings on the drive axles. What is the most likely root cause to investigate?
- Underinflated tires causing excess vibration
- Incorrect bearing preload adjustment or contaminated lubricant during installation (Correct answer)
- Excessive engine braking on downhill grades
- Mismatched tire brands on the drive axle
Correct answer: Incorrect bearing preload adjustment or contaminated lubricant during installation
Premature wheel bearing failure on Class 8 trucks is most commonly linked to improper preload during installation or lubricant contamination at the time of service.
Question 7: A fleet manager is analyzing total cost of ownership (TCO) for replacing versus repairing a vehicle with 180,000 miles that requires a $6,000 engine repair. Monthly depreciation on a replacement vehicle would be $450. What is the primary financial metric to calculate first?
- The vehicle's current trade-in value
- The break-even point in months between repair cost and replacement depreciation (Correct answer)
- The insurance premium difference between the old and new vehicle
- The fuel economy difference between the old and new vehicle
Correct answer: The break-even point in months between repair cost and replacement depreciation
Dividing the repair cost by the monthly depreciation of a replacement vehicle gives the break-even point, which is the core TCO metric for a repair-versus-replace decision.
NCT Case Analysis & Practical Application 3
A technician is diagnosing a fleet vehicle that stalls at idle but runs fine at higher RPMs.
Fuel pressure tests are within specification.
What system should be inspected next?