USPS Exam 943 Automotive Mechanic Technician
ACTUAL EXAM 2026/2027 | United States Postal
Service PSI Testing Services | 120 Question
Practice Assessment | Complete Solutions &
Explanations | Verified Q&A | Pass Guaranteed -
A+ Graded
Section 1: Engine Systems & Repair (Questions 1–22)
Q1. A 1994 Grumman LLV with the 2.5L Iron Duke engine stalls intermittently during hot weather
operation, particularly when idling at delivery stops. The check engine light is not illuminated.
Technician A suggests replacing the ignition control module. Technician B suggests testing the fuel
pump delivery volume and checking for vapor lock in the fuel lines. Which approach is most
diagnostically sound?
A. Technician A is correct; the ignition control module is a high-failure item on LLVs and should be
replaced prophylactically.
B. Both technicians are correct; replace the ignition module and the fuel pump simultaneously to
prevent callbacks.
C. Technician B is correct; the symptom pattern suggests fuel delivery issues or vapor lock, which
are common in LLVs due to underhood heat soak and fuel line routing. [CORRECT]
,D. Neither technician is correct; the throttle position sensor (TPS) is the most likely cause of hot
stalling.
Correct Answer: C
Rationale: (a) The diagnostic reasoning follows the symptom pattern: hot-weather stalling without
codes points toward fuel system vapor lock or insufficient pump volume, exacerbated by the LLV's
engine placement close to exhaust components and inadequate underhood ventilation during
extended idling. (b) Replacing the ignition control module (Option A) represents a "shotgun"
approach; while ICMs do fail, they typically set code 42 (GM OBD-I) or cause no-spark conditions
regardless of temperature. Option B's shotgun approach doubles the error. Option D's TPS failure
usually generates hesitation or codes, not specifically heat-related stall. (c) USPS VMH Section 3.2
and ASE A1 Task 5 (Fuel System Diagnosis) emphasize verifying fuel pressure and volume
(minimum 0.5 pint in 15 seconds for this TBI system) before replacing ignition components.
Q2. During a routine VMI (Vehicle Maintenance Inspection) on a 2019 Ram ProMaster 2500 with
the 3.6L Pentastar V6, the technician notices a P0304 misfire code stored in PCM memory. The
misfire is present under load but absent at idle. Freeze frame data shows fuel trim within
specification. Which diagnostic step should be performed first?
A. Replace all six spark plugs with copper core plugs as specified in the maintenance schedule.
B. Perform a cylinder compression test to verify mechanical integrity of cylinder #4.
C. Swap the ignition coil from cylinder #4 with cylinder #1 and verify if the misfire follows the coil.
[CORRECT]
D. Replace the fuel injector on cylinder #4, as coil-on-plug systems rarely fail on ProMaster
vehicles.
Correct Answer: C
,Rationale: (a) The under-load-only misfire pattern suggests an ignition secondary circuit weakness
that manifests under high spark demand. The isolation test (swapping coils) is the most efficient
first step to determine if the fault is coil-specific or constant (injector/compression). (b) Option A
wastes resources; if the issue is coil-related, new plugs won't resolve it. Option B is valid but
inefficient for an initial step; compression tests are invasive and time-consuming when ignition
components can be isolated non-intrusively. Option D represents a costly misdiagnosis;
coil-on-plug units have higher failure rates than injectors in this fleet. (c) USPS VMH ProMaster
Supplement Chapter 4 and ASE A8 Task 3 recommend ignition component substitution testing
before condemning fuel or mechanical systems.
Q3. An FFV (Flexible Fuel Vehicle) Ford E-450 Super Duty powered by a 5.4L V8 arrives with the
MIL illuminated and P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2)
stored. The vehicle has been operated primarily on E-85 for the past month. Long-term fuel trim is
+28% on both banks. Which component is the most likely root cause?
A. The fuel injectors are clogged from E-85 varnish deposits and require replacement.
B. The fuel composition sensor has failed, causing the PCM to calculate incorrect injector pulse
width for the actual alcohol content. [CORRECT]
C. The oxygen sensors have failed and are reading lean erroneously.
D. The fuel pump module assembly requires replacement due to decreased flow volume on ethanol
blends.
Correct Answer: B
Rationale: (a) The dual-bank lean condition with extreme positive fuel trim indicates the PCM is
adding maximum fuel but still detecting lean exhaust. Since the vehicle operates on variable
ethanol blends (E-0 to E-85), the fuel composition sensor (alcohol sensor) determines base
injector pulse width. If it under-reports alcohol content, the PCM provides gasoline-appropriate
injection timing for an ethanol-rich fuel, creating a severe lean condition. (b) Option A is unlikely
because E-85 is cleaner-burning than gasoline and less prone to varnish. Option C would typically
, show irrational sensor switching or heater circuit codes, not symmetric dual-bank lean. Option D
would affect both banks but usually manifests as pressure loss, not extreme trim without a P0087
code. (c) USPS Environmental Compliance Manual Section 5.3 and ASE A8 Task 1 emphasize
verifying fuel composition sensor operation (resistance range 50–150kΩ typical) before replacing
fuel system components on FFV platforms.
Q4. Using a scan tool on an LLV equipped with GM OBD-I (ALDL connector), the technician
retrieves Code 13 (O2 Sensor Circuit Open). The sensor has been replaced twice in three months.
Voltage at the O2 sensor signal wire (PCM pin B8) shows 0.45V with key-on-engine-off (KOEO),
but remains at 0.45V steady during engine operation instead of switching. What is the most likely
cause?
A. The replacement sensors are defective; use OEM sensors only.
B. There is an open circuit in the O2 sensor signal wire between the sensor and PCM, likely at the
C100 bulkhead connector. [CORRECT]
C. The PCM is defective and cannot process the O2 sensor signal.
D. The engine is running rich due to a stuck-open fuel pressure regulator, causing the sensor to
read maximum rich continuously.
Correct Answer: B
Rationale: (a) The 0.45V KOEO reading confirms the sensor heater and reference voltage are
intact, but the lack of switching during operation indicates the PCM is not receiving the signal. A
wiring open between the sensor and PCM would cause the PCM to see no signal variance,
interpreting it as a circuit fault. The C100 bulkhead connector on LLVs is notorious for
corrosion-induced opens. (b) Option A ignores the pattern of repeat failures pointing to an external
cause. Option C is a last-resort diagnosis; PCM failure would likely present multiple unrelated
codes. Option D is incorrect because a rich-running engine would still show O2 sensor switching
above 0.8V, not a static 0.45V. (c) USPS VMH LLV Electrical Diagram 8A-12 and ASE A6 Task 4
ACTUAL EXAM 2026/2027 | United States Postal
Service PSI Testing Services | 120 Question
Practice Assessment | Complete Solutions &
Explanations | Verified Q&A | Pass Guaranteed -
A+ Graded
Section 1: Engine Systems & Repair (Questions 1–22)
Q1. A 1994 Grumman LLV with the 2.5L Iron Duke engine stalls intermittently during hot weather
operation, particularly when idling at delivery stops. The check engine light is not illuminated.
Technician A suggests replacing the ignition control module. Technician B suggests testing the fuel
pump delivery volume and checking for vapor lock in the fuel lines. Which approach is most
diagnostically sound?
A. Technician A is correct; the ignition control module is a high-failure item on LLVs and should be
replaced prophylactically.
B. Both technicians are correct; replace the ignition module and the fuel pump simultaneously to
prevent callbacks.
C. Technician B is correct; the symptom pattern suggests fuel delivery issues or vapor lock, which
are common in LLVs due to underhood heat soak and fuel line routing. [CORRECT]
,D. Neither technician is correct; the throttle position sensor (TPS) is the most likely cause of hot
stalling.
Correct Answer: C
Rationale: (a) The diagnostic reasoning follows the symptom pattern: hot-weather stalling without
codes points toward fuel system vapor lock or insufficient pump volume, exacerbated by the LLV's
engine placement close to exhaust components and inadequate underhood ventilation during
extended idling. (b) Replacing the ignition control module (Option A) represents a "shotgun"
approach; while ICMs do fail, they typically set code 42 (GM OBD-I) or cause no-spark conditions
regardless of temperature. Option B's shotgun approach doubles the error. Option D's TPS failure
usually generates hesitation or codes, not specifically heat-related stall. (c) USPS VMH Section 3.2
and ASE A1 Task 5 (Fuel System Diagnosis) emphasize verifying fuel pressure and volume
(minimum 0.5 pint in 15 seconds for this TBI system) before replacing ignition components.
Q2. During a routine VMI (Vehicle Maintenance Inspection) on a 2019 Ram ProMaster 2500 with
the 3.6L Pentastar V6, the technician notices a P0304 misfire code stored in PCM memory. The
misfire is present under load but absent at idle. Freeze frame data shows fuel trim within
specification. Which diagnostic step should be performed first?
A. Replace all six spark plugs with copper core plugs as specified in the maintenance schedule.
B. Perform a cylinder compression test to verify mechanical integrity of cylinder #4.
C. Swap the ignition coil from cylinder #4 with cylinder #1 and verify if the misfire follows the coil.
[CORRECT]
D. Replace the fuel injector on cylinder #4, as coil-on-plug systems rarely fail on ProMaster
vehicles.
Correct Answer: C
,Rationale: (a) The under-load-only misfire pattern suggests an ignition secondary circuit weakness
that manifests under high spark demand. The isolation test (swapping coils) is the most efficient
first step to determine if the fault is coil-specific or constant (injector/compression). (b) Option A
wastes resources; if the issue is coil-related, new plugs won't resolve it. Option B is valid but
inefficient for an initial step; compression tests are invasive and time-consuming when ignition
components can be isolated non-intrusively. Option D represents a costly misdiagnosis;
coil-on-plug units have higher failure rates than injectors in this fleet. (c) USPS VMH ProMaster
Supplement Chapter 4 and ASE A8 Task 3 recommend ignition component substitution testing
before condemning fuel or mechanical systems.
Q3. An FFV (Flexible Fuel Vehicle) Ford E-450 Super Duty powered by a 5.4L V8 arrives with the
MIL illuminated and P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2)
stored. The vehicle has been operated primarily on E-85 for the past month. Long-term fuel trim is
+28% on both banks. Which component is the most likely root cause?
A. The fuel injectors are clogged from E-85 varnish deposits and require replacement.
B. The fuel composition sensor has failed, causing the PCM to calculate incorrect injector pulse
width for the actual alcohol content. [CORRECT]
C. The oxygen sensors have failed and are reading lean erroneously.
D. The fuel pump module assembly requires replacement due to decreased flow volume on ethanol
blends.
Correct Answer: B
Rationale: (a) The dual-bank lean condition with extreme positive fuel trim indicates the PCM is
adding maximum fuel but still detecting lean exhaust. Since the vehicle operates on variable
ethanol blends (E-0 to E-85), the fuel composition sensor (alcohol sensor) determines base
injector pulse width. If it under-reports alcohol content, the PCM provides gasoline-appropriate
injection timing for an ethanol-rich fuel, creating a severe lean condition. (b) Option A is unlikely
because E-85 is cleaner-burning than gasoline and less prone to varnish. Option C would typically
, show irrational sensor switching or heater circuit codes, not symmetric dual-bank lean. Option D
would affect both banks but usually manifests as pressure loss, not extreme trim without a P0087
code. (c) USPS Environmental Compliance Manual Section 5.3 and ASE A8 Task 1 emphasize
verifying fuel composition sensor operation (resistance range 50–150kΩ typical) before replacing
fuel system components on FFV platforms.
Q4. Using a scan tool on an LLV equipped with GM OBD-I (ALDL connector), the technician
retrieves Code 13 (O2 Sensor Circuit Open). The sensor has been replaced twice in three months.
Voltage at the O2 sensor signal wire (PCM pin B8) shows 0.45V with key-on-engine-off (KOEO),
but remains at 0.45V steady during engine operation instead of switching. What is the most likely
cause?
A. The replacement sensors are defective; use OEM sensors only.
B. There is an open circuit in the O2 sensor signal wire between the sensor and PCM, likely at the
C100 bulkhead connector. [CORRECT]
C. The PCM is defective and cannot process the O2 sensor signal.
D. The engine is running rich due to a stuck-open fuel pressure regulator, causing the sensor to
read maximum rich continuously.
Correct Answer: B
Rationale: (a) The 0.45V KOEO reading confirms the sensor heater and reference voltage are
intact, but the lack of switching during operation indicates the PCM is not receiving the signal. A
wiring open between the sensor and PCM would cause the PCM to see no signal variance,
interpreting it as a circuit fault. The C100 bulkhead connector on LLVs is notorious for
corrosion-induced opens. (b) Option A ignores the pattern of repeat failures pointing to an external
cause. Option C is a last-resort diagnosis; PCM failure would likely present multiple unrelated
codes. Option D is incorrect because a rich-running engine would still show O2 sensor switching
above 0.8V, not a static 0.45V. (c) USPS VMH LLV Electrical Diagram 8A-12 and ASE A6 Task 4
