NYC AUTO MECHANIC (DIESEL) DCAS CERTIFICATION EXAM
2026/2027 PRACTICE QUESTIONS AND STUDY GUIDE ACCURATE
EXAM ACTUAL QUESTIONS AND CORRECT DETAILED ANSWERS
WITH RATIONALES (100% CORRECT VERIFIED SOLUTIONS)
LATEST UPDATED VERSION 2026 EDITION |GUARANTEED SUCCESS
A+ (BRAND NEW!) |JUST RELEASED
1. A diesel engine is diagnosed with excessive black smoke, reduced power, and a
noticeable surge at high RPM. The fuel injection timing is found to be within
specifications. What is the MOST likely cause?
A. Air intake restriction
B. Excessive valve lash
C. Correct Answer: A. Air intake restriction (Inadequate air supply relative to fuel
injected creates a rich mixture, leading to incomplete combustion, black smoke,
and power loss. A surge can occur as the ECM or governor attempts to compensate
for the lack of air.)
2. During a cylinder contribution test on a heavy-duty diesel engine, one cylinder
shows a significant drop in RPM contribution. A relative compression test
indicates normal cranking compression. What is the MOST probable fault?
A. Broken piston ring
B. Correct Answer: B. Leaking injector (A leaking injector can cause a rich
condition in that cylinder, reducing its power contribution during the test despite
normal mechanical compression. The excess fuel washes the cylinder and disrupts
combustion.)
,3. When performing a Diesel Particulate Filter (DPF) forced regeneration, which of
the following MUST be observed to prevent a thermal event?
A. The vehicle must be parked on a level surface.
B. Correct Answer: B. Monitor exhaust temperatures (During active regeneration,
exhaust temperatures can exceed 1,200°F. Constant monitoring is critical to
prevent damage to the system and the vehicle, and to ensure the regeneration is
progressing correctly.)
4. An electronically controlled diesel engine fails to start. The scan tool indicates a
loss of communication with the ECM and the engine protection system is active.
What is the FIRST step in diagnosing the no-start?
A. Check the starter relay.
B. Correct Answer: B. Check engine ground and battery connections (A loss of
communication often indicates a power or ground issue. Verifying the integrity of
battery connections and main engine grounds is the primary step before
condemning the ECM.)
5. What is the purpose of the "crankcase pressure" or "blow-by" test?
A. To measure the oil pressure at idle.
B. Correct Answer: B. To assess the condition of piston rings (Excessive crankcase
pressure, measured as blow-by, directly indicates leakage past the piston rings,
allowing combustion gases into the crankcase, which suggests cylinder wear.)
6. In a HEUI fuel injection system, what component provides the high-pressure oil
required to actuate the injectors?
,A. High-pressure fuel pump
B. Correct Answer: B. High-pressure oil pump (HPOP) (The HEUI system uses oil
pressure rather than fuel pressure to hydraulically amplify the injection pressure.
The HPOP is the primary component responsible for this, typically producing 500-
3,000 PSI.)
7. A diesel engine is overheating. The radiator cap is cool, and the hoses are soft.
The water pump is turning, but the coolant in the radiator is stationary. What is the
MOST likely cause?
A. Faulty thermostat
B. Correct Answer: B. Stuck closed thermostat (If the thermostat is stuck closed,
coolant flow is blocked. The pump may spin, but it cannot circulate coolant
through the radiator, leading to the described symptoms.)
8. During a brake inspection on a heavy-duty truck, a mechanic finds a seized
automatic slack adjuster. What is the proper action?
A. Lubricate the adjuster and free it manually.
B. Correct Answer: B. Replace the slack adjuster (Seized automatic slack adjusters
cannot be reliably repaired or lubricated to return to service. Replacement is the
mandatory repair per FMCSA regulations to ensure proper braking function.)
9. An Allison automatic transmission is slipping on an upshift. The fluid level is
correct and has a burnt smell. What is the MOST likely internal fault?
A. Malfunctioning TCM.
B. Correct Answer: B. Worn clutch plates (A burnt smell in automatic transmission
fluid indicates overheating of the friction material, typically from worn clutch
plates slipping under load, which requires a rebuild or replacement.)
, 10. What does the "C" in the DTC code P0171 indicate in the context of OBD-II?
A. Engine
B. Correct Answer: B. Powertrain (The first character of a generic OBD-II code
"P" indicates Powertrain. The "0" indicates a generic code, and "171" refers to a
system too lean condition.)
11. When checking the air gap for an electromagnetic speed sensor on an ABS
system, which tool is required?
A. Torque wrench
B. Correct Answer: B. Feeler gauge (The air gap is a physical measurement
between the sensor tip and the tone ring. A feeler gauge is the standard tool for this
precision check.)
12. A driveline vibration occurs only during acceleration. Which component is the
MOST likely cause?
A. Out-of-balance tire
B. Correct Answer: B. Worn universal joint (Vibration during acceleration is a
classic symptom of a worn or seized u-joint. An out-of-balance tire usually vibrates
at a specific speed, and a bad carrier bearing often vibrates during deceleration.)
13. What is the function of a diesel oxidation catalyst (DOC)?
A. To trap soot particles.
2026/2027 PRACTICE QUESTIONS AND STUDY GUIDE ACCURATE
EXAM ACTUAL QUESTIONS AND CORRECT DETAILED ANSWERS
WITH RATIONALES (100% CORRECT VERIFIED SOLUTIONS)
LATEST UPDATED VERSION 2026 EDITION |GUARANTEED SUCCESS
A+ (BRAND NEW!) |JUST RELEASED
1. A diesel engine is diagnosed with excessive black smoke, reduced power, and a
noticeable surge at high RPM. The fuel injection timing is found to be within
specifications. What is the MOST likely cause?
A. Air intake restriction
B. Excessive valve lash
C. Correct Answer: A. Air intake restriction (Inadequate air supply relative to fuel
injected creates a rich mixture, leading to incomplete combustion, black smoke,
and power loss. A surge can occur as the ECM or governor attempts to compensate
for the lack of air.)
2. During a cylinder contribution test on a heavy-duty diesel engine, one cylinder
shows a significant drop in RPM contribution. A relative compression test
indicates normal cranking compression. What is the MOST probable fault?
A. Broken piston ring
B. Correct Answer: B. Leaking injector (A leaking injector can cause a rich
condition in that cylinder, reducing its power contribution during the test despite
normal mechanical compression. The excess fuel washes the cylinder and disrupts
combustion.)
,3. When performing a Diesel Particulate Filter (DPF) forced regeneration, which of
the following MUST be observed to prevent a thermal event?
A. The vehicle must be parked on a level surface.
B. Correct Answer: B. Monitor exhaust temperatures (During active regeneration,
exhaust temperatures can exceed 1,200°F. Constant monitoring is critical to
prevent damage to the system and the vehicle, and to ensure the regeneration is
progressing correctly.)
4. An electronically controlled diesel engine fails to start. The scan tool indicates a
loss of communication with the ECM and the engine protection system is active.
What is the FIRST step in diagnosing the no-start?
A. Check the starter relay.
B. Correct Answer: B. Check engine ground and battery connections (A loss of
communication often indicates a power or ground issue. Verifying the integrity of
battery connections and main engine grounds is the primary step before
condemning the ECM.)
5. What is the purpose of the "crankcase pressure" or "blow-by" test?
A. To measure the oil pressure at idle.
B. Correct Answer: B. To assess the condition of piston rings (Excessive crankcase
pressure, measured as blow-by, directly indicates leakage past the piston rings,
allowing combustion gases into the crankcase, which suggests cylinder wear.)
6. In a HEUI fuel injection system, what component provides the high-pressure oil
required to actuate the injectors?
,A. High-pressure fuel pump
B. Correct Answer: B. High-pressure oil pump (HPOP) (The HEUI system uses oil
pressure rather than fuel pressure to hydraulically amplify the injection pressure.
The HPOP is the primary component responsible for this, typically producing 500-
3,000 PSI.)
7. A diesel engine is overheating. The radiator cap is cool, and the hoses are soft.
The water pump is turning, but the coolant in the radiator is stationary. What is the
MOST likely cause?
A. Faulty thermostat
B. Correct Answer: B. Stuck closed thermostat (If the thermostat is stuck closed,
coolant flow is blocked. The pump may spin, but it cannot circulate coolant
through the radiator, leading to the described symptoms.)
8. During a brake inspection on a heavy-duty truck, a mechanic finds a seized
automatic slack adjuster. What is the proper action?
A. Lubricate the adjuster and free it manually.
B. Correct Answer: B. Replace the slack adjuster (Seized automatic slack adjusters
cannot be reliably repaired or lubricated to return to service. Replacement is the
mandatory repair per FMCSA regulations to ensure proper braking function.)
9. An Allison automatic transmission is slipping on an upshift. The fluid level is
correct and has a burnt smell. What is the MOST likely internal fault?
A. Malfunctioning TCM.
B. Correct Answer: B. Worn clutch plates (A burnt smell in automatic transmission
fluid indicates overheating of the friction material, typically from worn clutch
plates slipping under load, which requires a rebuild or replacement.)
, 10. What does the "C" in the DTC code P0171 indicate in the context of OBD-II?
A. Engine
B. Correct Answer: B. Powertrain (The first character of a generic OBD-II code
"P" indicates Powertrain. The "0" indicates a generic code, and "171" refers to a
system too lean condition.)
11. When checking the air gap for an electromagnetic speed sensor on an ABS
system, which tool is required?
A. Torque wrench
B. Correct Answer: B. Feeler gauge (The air gap is a physical measurement
between the sensor tip and the tone ring. A feeler gauge is the standard tool for this
precision check.)
12. A driveline vibration occurs only during acceleration. Which component is the
MOST likely cause?
A. Out-of-balance tire
B. Correct Answer: B. Worn universal joint (Vibration during acceleration is a
classic symptom of a worn or seized u-joint. An out-of-balance tire usually vibrates
at a specific speed, and a bad carrier bearing often vibrates during deceleration.)
13. What is the function of a diesel oxidation catalyst (DOC)?
A. To trap soot particles.
