Smog Repair Technician Exam Questions and Correct Answers.

Smog Repair Technician Exam Questions
and Correct Answers
Question 1
During a smog check, when performing an ignition timing check, the technician is
instructed to increase the engine's RPM to verify the timing. If the engine is idling at
800 RPM, by how many RPM should the technician increase the engine speed to
properly check the timing?
Correct Answer
100 rpm



Question 2
Which pin on the OBD-II connector is used for chassis ground?
Correct Answer
Pin 4



Question 3
A cylinder shows evenly low compression in all cylinders (around 110 psi), but they
are within 5% of each other. What does this most likely indicate?
Correct Answer
Worn piston rings



Question 4
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?
Correct Answer
Leaking intake manifold gasket




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,Question 5
A car fails smog due to incomplete monitors and a "no communication" DTC. What
should a technician do FIRST when diagnosing the issue?
Correct Answer
Inspect Pin 4 and Pin 5 for ground and Pin 16 for power



Question 6
A car fails smog with these emissions readings:
HC: 90 ppm
CO: 0.1%
CO₂: 14.8%
O₂: 3.5%
NOx: 2400 ppm
What is the most likely cause of this failure?
Correct Answer
Vacuum leak



Question 7
What is the normal voltage range for a properly working upstream oxygen sensor?
Correct Answer
0.1V - 0.9V



Question 8
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?
Correct Answer
EGR system



Question 9
A vehicle shows LTFT at -20% and STFT at -10%. What component failure could cause
this and result in smog failure?
Correct Answer
clogged PCV valve




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,Question 10
A vehicle's compression test shows the following:
Cylinder 1: 135 psi
Cylinder 2: 135 psi
Cylinder 3: 70 psi
Cylinder 4: 130 psi
What is the most likely cause of the low compression in Cylinder 3?
Correct Answer
Leaking intake valve



Question 11
A vehicle has failed a smog check due to high NOx emissions. The technician needs to
check real-time sensor data, including fuel trims and oxygen sensor readings, to
diagnose the problem. Which mode should they use?
Correct Answer
Mode 1



Question 12
If a vehicle fails a smog check due to a communication error with the scan tool, which
OBD-II pin should be checked first for proper power?
Correct Answer
Pin 16



Question 13
What is the primary function of the secondary air injection system?
Correct Answer
Heat up the catalytic converter faster



Question 14
A vehicle has an LTFT of -18%. What is the most likely cause?
Correct Answer
leaking fuel pressure regulator




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, Question 15
Which of the following is an advantage of a leakdown test over a compression test?
Correct Answer
A leakdown test identifies the source of cylinder leakage, while a compression test
does not



Question 16
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?
Correct Answer
Pin 4



Question 17
A downstream O2 sensor fluctuating just like the upstream sensor usually indicates:
Correct Answer
A failing catalytic converter



Question 18
Fuel trims within these ranges are considered normal
Correct Answer
+/-10%



Question 19
A compression test on a four-cylinder engine yields the following results:
Cylinder 1: 150 psi
Cylinder 2: 140 psi
Cylinder 3: 30 psi
Cylinder 4: 130 psi
What is the most likely cause of the low compression in Cylinder 3?
Correct Answer
Poor valve seating




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