Question 1

Technician A says when CO goes up, O₂ goes down.Technician B says at the stoichiometric point [14.7:1], HC and CO will be at their lowest levels and CO₂ will be at its highest. Who is correct?

Accepted Answer

Both technicians are correct. Technician A is correct because when CO (carbon monoxide) levels increase, it indicates a rich fuel mixture, which leads to a decrease in oxygen sensor voltage (O₂ refers to an oxygen sensor reading). Technician B is correct because at the stoichiometric air-fuel ratio of 14.7:1, combustion is most efficient, resulting in the lowest levels of HC (hydrocarbons) and CO, and the oxygen sensor voltage will be at its optimal level, indicating a balanced air-fuel mixture.

Question 2

Technician A says an I/M 240 test is four minutes long. Technician B says the test cycle includes acceleration, deceleration, and cruising on an actual road test. Who is correct?

Accepted Answer

According to EPA guidelines, an I/M 240 test should last four minutes. Technician B is incorrect, as an I/M 240 test is conducted on a dynamometer, not on an actual road test.

Question 3

Technician A says cylinder misfiring causes a significant increase in HC emissions. Technician B says a dirty air filter may cause high CO emissions. Who is correct?

Accepted Answer

Technician A is correct. A cylinder misfire can cause unburned fuel to enter the exhaust system, leading to a spike in hydrocarbon (HC) emissions. 
Technician B is also correct. A dirty air filter can cause a rich air-fuel mixture to be delivered to the engine, resulting in high carbon monoxide (CO) emissions.

Question 4

Excessively high HC emissions may be caused by ______.

Accepted Answer

Excessively high hydrocarbon (HC) emissions can be caused by ignition system misfiring and improper ignition timing, as both can lead to incomplete combustion of the air-fuel mixture, resulting in higher HC emissions.

Question 5

A typical four-gas exhaust analyzer does not measure ____ emissions.

Accepted Answer

The option "NO_X" in choice D is likely a typo and does not make sense. However, the other three options are valid pollutants measured by a typical four-gas exhaust analyzer. Therefore, the correct answer is D as it is the only option that does not contain a valid pollutant measured by the device.

Question 6

Technician A says the AIRB valve directs secondary air either to the intake manifold or to the catalytic converter. Technician B says with the engine at normal operating temperature, the AIR system sometimes goes back into upstream mode with the engine idling. Who is correct?

Accepted Answer

The answer of Technician A says the AIRB valve directs...

Question 7

Excessively high CO emissions may be caused by a ______.

Accepted Answer

Higher than normal fuel pressure can cause excessively high CO emissions due to an overly rich air/fuel mixture.

Question 8

Crankcase pressure forcing crankcase gases through the clean air hose and into the air cleaner is being discussed. Technician A says the cause may be an excessive amount of blowby. Technician B says the cause may be a restricted PCV valve. Who is correct?

Accepted Answer

Both technicians are partially correct. Excessive blowby can cause crankcase pressure and force gases through the clean air hose into the air cleaner. A restricted PCV valve can also cause crankcase pressure and force gases through the clean air hose into the air cleaner. Therefore, both technicians A and B are correct.

Question 9

The customer complains of a loss of power at highway speeds and a rough running engine at all speeds. A five-gas analyzer reveals these measurements at 2,500 rpm: HC = 700 ppm, CO = 0.2%, CO₂ = 9.2%, O₂ = 6.2%, and NO_X = 330 ppm. What is the most likely cause?

Accepted Answer

The high HC (hydrocarbons) level indicates unburned fuel, which, along with the normal CO (carbon monoxide) level, suggests incomplete combustion rather than a rich or lean mixture. This scenario is indicative of a lean misfire, where the engine misfires due to a lean fuel mixture, leading to rough running and loss of power.

Question 10

Technician A says a defective throttle position (TP) sensor will not affect the EGR valve operation. Technician B says a defective engine coolant temperature (ECT) sensor will not affect the EGR valve operation. Who is correct?

Accepted Answer

A defective throttle position sensor can affect EGR valve operation because the EGR system may use throttle position information to regulate exhaust gas recirculation. Similarly, a defective engine coolant temperature sensor can affect EGR valve operation because the EGR system may also use coolant temperature information to determine when and how much to recirculate.

Question 11

While discussing catalytic converter diagnosis, Technician A says a delta temperature test should be conducted. Technician B says a good converter is evident by low amounts of CO₂ in the tailpipe exhaust. Who is correct?

Accepted Answer

Technician A is correct. A delta temperature test is one of the diagnostic methods to identify a malfunctioning catalytic converter. The test involves measuring the temperature difference between the inlet and the outlet of the converter. If the temperature at the outlet is significantly lower than the temperature at the inlet, it indicates that the converter is not working as it should. Technician B is incorrect. CO₂ is not a valid measurement. It is likely a typo or an error. A good converter can be identified by measuring the emissions in the tailpipe exhaust, but the measurements should be of valid pollutants such as CO, NOx, and HC.

Question 12

A vehicle is in the shop because it failed an emissions test due to excessive carbon monoxide [CO]. The fault was a bad oxygen sensor that was easily replaced. Technician A says that the EGR system should be checked before the vehicle is retested. Technician B says the vehicle should be tested with a five-gas analyzer before it is retested. Who is correct?

Accepted Answer

Both technicians are correct. Checking the EGR (Exhaust Gas Recirculation) system can help ensure it is functioning properly and not contributing to the emissions problem. Using a five-gas analyzer can provide a comprehensive view of the vehicle's emissions, including CO, to ensure the issue has been resolved before retesting.

Question 13

When the engine is running at peak efficiency, the only gas that is desirable to be at its peak level is ______.

Accepted Answer

The answer of When the engine is running at peak...

Question 14

Technician A says CO emissions increase as the air/fuel ratio becomes richer. Technician B says CO emissions increase as the air/fuel ratio becomes leaner. Who is correct?

Accepted Answer

CO emissions increase as the air/fuel mixture becomes richer because more carbon monoxide is produced when there is not enough oxygen to fully combust the fuel, leading to incomplete combustion.

Question 15

Technician A says a defective PCV valve may cause rough idle operation. Technician B says the PCV system depends upon a properly sealed engine to perform correctly. Who is correct?

Accepted Answer

Technician A is correct because a defective PCV valve can cause rough idle due to an imbalance in the air/fuel mixture or vacuum pressure. Technician B is also correct because the PCV system relies on a properly sealed engine to ensure that the crankcase gases are properly ventilated. Any leaks or breaches in the engine sealing can cause the PCV system to malfunction. Therefore, both A and B are correct.

Question 16

Technician A says if the EVAP system is purging vapors from the charcoal canister while the engine is idling, rough engine operation will occur. Technician B says the system should not have more than 0.5 liter of flow during the 240 seconds of an I/M 240 test. Who is correct?

Accepted Answer

While both technicians are discussing the EVAP system, only Technician A is correct. Purging vapors from the charcoal canister during idle can cause rough engine operation. Technician B's statement is incorrect as the system should not have more than 0.5 grams of flow during the 240 seconds of an I/M 240 test, not liters.

Question 17

Technician A says a pressure gauge can be inserted into the oxygen sensor bore, and back pressure caused by the catalytic converter can be measured. Technician B says a vacuum gauge can be used to identify a restricted converter. Who is correct?

Accepted Answer

Both technicians are correct. Technician A is correct that a pressure gauge can be inserted into the oxygen sensor bore to measure back pressure caused by the catalytic converter. Technician B is correct that a vacuum gauge can be used to identify a restricted converter. These are both valid diagnostic methods for identifying issues with the catalytic converter.

Question 18

Technician A says if the EGR valve does not open, the engine may emit excessive amounts of HC. Technician B says if the EGR valve does not open, the engine may detonate on acceleration. Who is correct?

Accepted Answer

Technician B is correct because if the EGR (Exhaust Gas Recirculation) valve does not open, it can lead to increased combustion temperatures, which in turn can cause engine detonation or "knocking" during acceleration. The EGR valve's purpose is to recirculate a portion of the exhaust gases back into the combustion chamber to cool the combustion temperature, thereby reducing NOx emissions and preventing detonation. Technician A's statement about excessive HC (hydrocarbons) emissions is incorrect in this context; HC emissions are more directly related to incomplete combustion or issues with the ignition system, not the operation of the EGR valve.

Question 19

Technician A says retrieving DTCs from the control system is the best beginning for diagnosing emission problems. Technician B says because a single emissions system component failure can set multiple DTCs, codes should be used only to confirm the effectiveness of the repairs. Who is correct?

Accepted Answer

The answer of Technician A says retrieving DTCs from the...

Question 20

To monitor the vapor purge in most systems, the PCM changes the duty cycle of the purge solenoid while monitoring the ______.

Accepted Answer

The STFT (short term fuel trim) is used by the PCM (powertrain control module) to monitor the effectiveness of the vapor purge system. By changing the duty cycle of the purge solenoid while monitoring the STFT, the PCM can determine if the system is working properly. The LTFT (long term fuel trim), MAP voltage, and MAF voltage are not directly related to the vapor purge system.

Quiz 34: Emission Control Diagnosis and Service

Technician A says when CO goes up, O2 goes down.Technician B says at the stoichiometric point [14.7:1], HC and CO will be at their lowest levels and CO2 will be at its highest. Who is correct?