SMOG REPAIR TECHNICIAN EXAM
QUESTIONS AND ANSWERS WITH
COMPLETE SOLUTIONS 100%
CORRECT RATED A+
1. Handling Incomplete Monitors
Q: A car came in for a smog check and failed due to incomplete monitors.
What resource should the technician use? A: The On-Board Diagnostic Test
Reference (OBDTR).
Why: The OBDTR provides specific manufacturer "Drive Cycle"
procedures. These drive cycles are required to reset readiness monitors that
have been cleared (e.g., after a battery disconnect or code clear).
2. Analyzing Fuel Trim (FT) Data
Fuel trims are percentages that indicate how much the Engine Control Module
(ECM) is adjusting fuel injection to compensate for a non-ideal air-fuel ratio.
Positive (+) Trim: The ECM is adding fuel (the engine is running lean).
Negative (-) Trim: The ECM is subtracting fuel (the engine is running
rich).
Q: A car fails a smog check due to high NOx emissions and has an LTFT
(Long Term Fuel Trim) of +25%. What is the MOST likely cause? A: A
clogged fuel injector.
Logic: A clogged injector restricts fuel delivery, forcing the computer to add
fuel (high positive trim) to compensate. An engine running lean often results
in higher combustion temperatures, which directly leads to an increase in
NOx (Oxides of Nitrogen) emissions.
Q: A vehicle with a P0174 (System Too Lean - Bank 2) code is experiencing
rough idle and hesitation under acceleration. What is the MOST likely cause?
A: A leaking intake manifold gasket.
, Logic: A vacuum leak (like a bad gasket) introduces unmetered air into the
engine. This causes a "lean" condition because the computer is not
accounting for that extra oxygen, leading to the misfire or hesitation felt by
the driver.
Q: A technician is diagnosing a vehicle with an STFT (Short Term Fuel Trim)
of +18% and an LTFT of +30%. Which of the following should be checked
FIRST? A: Fuel pressure.
Logic: When both STFT and LTFT are high, the engine is drastically
starved of fuel. Checking fuel pressure confirms if the delivery system (fuel
pump, filter, or regulator) is capable of supplying the engine, as this is a
"global" issue affecting all cylinders.
3. Lean Conditions and NOx
Q: Which of the following could cause a car to run lean and fail a smog check
due to high NOx emissions? A: A faulty Mass Air Flow (MAF) sensor.
Logic: If the MAF sensor under-reports the amount of air entering the
engine, the computer will inject too little fuel. This creates a lean air-fuel
mixture, which causes combustion temperatures to soar, resulting in high
NOx readings during the smog check.
3 multiple choice options
Which of the following conditions can cause a vehicle to fail a smog check due to
a P0420 (Catalyst System Efficiency Below Threshold) code?
Rich air-fuel mixture
3 multiple choice options
A technician inspects a vehicle that failed smog due to high NOx emissions and
finds the catalytic converter is intact. What should be checked NEXT?
EGR system
3 multiple choice options
, A vehicle has a P0430 (Catalyst System Efficiency Below Threshold - Bank 2)
code. Which of the following should be checked BEFORE replacing the catalytic
converter?
Fuel trim values
3 multiple choice options
A technician connects a scan tool to a vehicle's OBD-II port, but there is no power.
The ground circuit is verified as good. Which pin should be checked next for
power?
Pin 16
3 multiple choice options
A vehicle using CAN (Controller Area Network) communication fails a smog test
due to an OBD communication error. Which two pins should be checked for
proper CAN signal operation?
Pins 6 and 14
3 multiple choice options
A vehicle fails a smog test due to OBD-II readiness monitors not completing. The
scan tool shows no connection to the ECU. Which OBD-II pin is used for primary
ground in almost all vehicles?
Pin 4
3 multiple choice options
Which OBD-II port pin is responsible for providing power to the scan tool on most
vehicles?
Pin 16
3 multiple choice options
Pin 16
12V power
3 multiple choice options
Pin 6
QUESTIONS AND ANSWERS WITH
COMPLETE SOLUTIONS 100%
CORRECT RATED A+
1. Handling Incomplete Monitors
Q: A car came in for a smog check and failed due to incomplete monitors.
What resource should the technician use? A: The On-Board Diagnostic Test
Reference (OBDTR).
Why: The OBDTR provides specific manufacturer "Drive Cycle"
procedures. These drive cycles are required to reset readiness monitors that
have been cleared (e.g., after a battery disconnect or code clear).
2. Analyzing Fuel Trim (FT) Data
Fuel trims are percentages that indicate how much the Engine Control Module
(ECM) is adjusting fuel injection to compensate for a non-ideal air-fuel ratio.
Positive (+) Trim: The ECM is adding fuel (the engine is running lean).
Negative (-) Trim: The ECM is subtracting fuel (the engine is running
rich).
Q: A car fails a smog check due to high NOx emissions and has an LTFT
(Long Term Fuel Trim) of +25%. What is the MOST likely cause? A: A
clogged fuel injector.
Logic: A clogged injector restricts fuel delivery, forcing the computer to add
fuel (high positive trim) to compensate. An engine running lean often results
in higher combustion temperatures, which directly leads to an increase in
NOx (Oxides of Nitrogen) emissions.
Q: A vehicle with a P0174 (System Too Lean - Bank 2) code is experiencing
rough idle and hesitation under acceleration. What is the MOST likely cause?
A: A leaking intake manifold gasket.
, Logic: A vacuum leak (like a bad gasket) introduces unmetered air into the
engine. This causes a "lean" condition because the computer is not
accounting for that extra oxygen, leading to the misfire or hesitation felt by
the driver.
Q: A technician is diagnosing a vehicle with an STFT (Short Term Fuel Trim)
of +18% and an LTFT of +30%. Which of the following should be checked
FIRST? A: Fuel pressure.
Logic: When both STFT and LTFT are high, the engine is drastically
starved of fuel. Checking fuel pressure confirms if the delivery system (fuel
pump, filter, or regulator) is capable of supplying the engine, as this is a
"global" issue affecting all cylinders.
3. Lean Conditions and NOx
Q: Which of the following could cause a car to run lean and fail a smog check
due to high NOx emissions? A: A faulty Mass Air Flow (MAF) sensor.
Logic: If the MAF sensor under-reports the amount of air entering the
engine, the computer will inject too little fuel. This creates a lean air-fuel
mixture, which causes combustion temperatures to soar, resulting in high
NOx readings during the smog check.
3 multiple choice options
Which of the following conditions can cause a vehicle to fail a smog check due to
a P0420 (Catalyst System Efficiency Below Threshold) code?
Rich air-fuel mixture
3 multiple choice options
A technician inspects a vehicle that failed smog due to high NOx emissions and
finds the catalytic converter is intact. What should be checked NEXT?
EGR system
3 multiple choice options
, A vehicle has a P0430 (Catalyst System Efficiency Below Threshold - Bank 2)
code. Which of the following should be checked BEFORE replacing the catalytic
converter?
Fuel trim values
3 multiple choice options
A technician connects a scan tool to a vehicle's OBD-II port, but there is no power.
The ground circuit is verified as good. Which pin should be checked next for
power?
Pin 16
3 multiple choice options
A vehicle using CAN (Controller Area Network) communication fails a smog test
due to an OBD communication error. Which two pins should be checked for
proper CAN signal operation?
Pins 6 and 14
3 multiple choice options
A vehicle fails a smog test due to OBD-II readiness monitors not completing. The
scan tool shows no connection to the ECU. Which OBD-II pin is used for primary
ground in almost all vehicles?
Pin 4
3 multiple choice options
Which OBD-II port pin is responsible for providing power to the scan tool on most
vehicles?
Pin 16
3 multiple choice options
Pin 16
12V power
3 multiple choice options
Pin 6
