CSLB General Building (B) — All Questions

An oxygen (O2) sensor in the exhaust provides feedback to the PCM about:

• a.Engine oil level
• b.Coolant temperature
• c.The air-fuel ratio (rich or lean)✓
• d.Ignition timing advance

The oxygen sensor measures residual oxygen in the exhaust, telling the PCM whether the mixture is rich or lean so it can adjust fuel trim. A lazy or failed sensor degrades fuel control and emissions. Wide-band sensors report the actual ratio rather than just rich/lean.

A diagnostic trouble code (DTC) of P0301 indicates:

• a.A misfire detected in cylinder number 1✓
• b.A lean condition on bank 1
• c.A faulty coolant sensor
• d.An EVAP leak

P0301 is a cylinder-specific misfire code, with the last digit identifying cylinder 1. P0300 indicates a random/multiple-cylinder misfire. Causes include ignition, fuel, or mechanical (compression) problems in that cylinder.

Technician A says a vacuum leak causes a rich condition. Technician B says a vacuum leak adds unmetered air and causes a lean condition, especially at idle. Who is correct?

• a.Technician A only
• b.Technician B only✓
• c.Both A and B
• d.Neither A nor B

A vacuum leak lets extra air enter after the mass airflow sensor, leaning the mixture and often causing a rough idle and positive fuel trim. Technician B is correct. Smoke testing helps locate intake leaks.

Technician A says a positive long-term fuel trim of +20 percent means the PCM is adding fuel to correct a lean condition. Technician B says positive trim means the engine is running rich. Who is correct?

• a.Technician A only✓
• b.Both A and B
• c.Technician B only
• d.Neither A nor B

Positive fuel trim means the computer is adding fuel to correct a lean condition from a vacuum leak, low fuel pressure, or a dirty MAF. Technician B is wrong because rich conditions produce negative trims. Comparing short- and long-term trims helps pinpoint the fault.

Technician A says a rich air-fuel mixture can raise tailpipe CO and HC. Technician B says the oxygen sensor helps the PCM keep the mixture near stoichiometric. Who is correct?

• a.Technician A only
• b.Technician B only
• c.Both A and B✓
• d.Neither A nor B

A rich mixture leaves excess fuel that raises carbon monoxide and hydrocarbons, and the oxygen sensor feedback lets the PCM hold the mixture near the ideal ratio. Both technicians are correct. A lazy sensor degrades fuel control and emissions.

Technician A says the MAF sensor measures the mass of air entering the engine so the PCM can meter fuel. Technician B says a dirty MAF can cause hesitation and fuel-trim errors. Who is correct?

• a.Neither A nor B
• b.Technician A only
• c.Technician B only
• d.Both A and B✓

The MAF sensor measures intake air mass so the PCM meters the correct fuel quantity, and a dirty or faulty MAF causes incorrect fueling, hesitation, and trim errors. Both technicians are correct. Diagnosis includes cleaning or replacing it and checking for unmetered air leaks.

A P0420 code (catalyst efficiency below threshold) most often indicates:

• a.A degraded catalytic converter or an O2 sensor issue✓
• b.A slipping transmission
• c.A worn wheel bearing
• d.A faulty ABS sensor

P0420 sets when the downstream O2 sensor signal mirrors the upstream too closely, suggesting the converter is no longer storing oxygen effectively. Causes include a worn converter, exhaust leaks, or faulty sensors. Underlying misfires or rich running can damage the converter and must be addressed first.

In a modern coil-on-plug ignition system, each coil:

• a.Powers all cylinders at once
• b.Is triggered by a mechanical distributor
• c.Stores fuel for injection
• d.Fires the spark plug directly on its cylinder, controlled by the PCM✓

Coil-on-plug systems place an individual coil at each spark plug, fired by the PCM through the ignition driver, eliminating plug wires and the distributor. This improves spark control and misfire diagnosis. A failed coil typically causes a cylinder-specific misfire code.

A knock sensor allows the PCM to:

• a.Measure exhaust oxygen
• b.Detect detonation and retard ignition timing to protect the engine✓
• c.Control the fuel pump
• d.Read vehicle speed

The knock sensor detects the vibration of detonation so the PCM can retard timing and prevent engine damage. Persistent knock can come from low-octane fuel, carbon buildup, or overheating. A faulty sensor may cause reduced performance or a code.

The EVAP system is designed to:

• a.Cool the intake charge
• b.Filter engine oil
• c.Capture and burn fuel vapors from the tank rather than venting them✓
• d.Increase fuel pressure

The evaporative emission system stores fuel vapors in a charcoal canister and later purges them into the intake to be burned, preventing release to the atmosphere. A loose or faulty gas cap is a common cause of small-leak codes. It is tested for leaks during OBD-II monitors.

A loose or missing fuel cap most commonly sets which type of code?

• a.An EVAP system leak code (such as P0455 or P0457)✓
• b.A misfire code
• c.A coolant temperature code
• d.An ABS code

A poorly sealing fuel cap lets vapors escape and triggers an EVAP leak code, often the first thing to check for such faults. Tightening or replacing the cap and clearing the code frequently resolves it. Smoke testing finds larger leaks in the system.

Technician A says low fuel pressure can cause a lean misfire and hesitation. Technician B says a clogged fuel filter or weak pump can lower fuel pressure. Who is correct?

• a.Technician A only
• b.Technician B only
• c.Neither A nor B
• d.Both A and B✓

Insufficient fuel pressure leans the mixture, causing misfire, hesitation, and lack of power, and a restricted filter or failing pump is a common cause. Both technicians are correct. A fuel-pressure test and volume check confirm delivery problems.

The throttle position sensor (TPS) informs the PCM about:

• a.Exhaust temperature
• b.Throttle plate angle (driver demand)✓
• c.Coolant level
• d.Battery voltage

The TPS reports how far the throttle is open so the PCM can adjust fuel, timing, and transmission strategy. A faulty or dead-spot TPS causes hesitation, surging, or erratic shifts. Its signal is checked with a scan tool or by watching voltage sweep.

A vehicle fails an emissions test for high hydrocarbons (HC). This is most commonly associated with:

• a.Excessive coolant
• b.Low tire pressure
• c.Unburned fuel from misfire, ignition, or rich-condition problems✓
• d.A worn wheel bearing

High HC indicates unburned fuel escaping into the exhaust, typically from misfires, ignition faults, or an overly rich mixture. Diagnosing the misfire or fuel-control fault reduces HC. A failing catalytic converter can also raise tailpipe HC.

The PCM uses the crankshaft position sensor primarily to:

• a.Determine engine speed and piston position for ignition and fuel timing✓
• b.Measure intake air temperature
• c.Read exhaust oxygen
• d.Detect wheel slip

The crankshaft position sensor tells the PCM engine rpm and crank position so it can time spark and injection. Loss of this signal usually prevents the engine from starting. A cam/crank correlation code indicates timing has slipped between the two.

On an OBD-II vehicle, the data link connector (DLC) used to retrieve codes is standardized as a:

• a.9-pin round plug under the hood
• b.2-pin connector at the battery
• c.4-pin trailer connector
• d.16-pin connector, usually under the dash✓

OBD-II mandates a standardized 16-pin DLC, typically located under the driver's side dash, for scan-tool access. This standard allows generic tools to read codes and data across manufacturers. Certain pins carry power, ground, and communication signals.

The exhaust gas recirculation (EGR) system reduces which emission by lowering combustion temperature?

• a.Carbon monoxide (CO)
• b.Oxides of nitrogen (NOx)✓
• c.Hydrocarbons (HC)
• d.Carbon dioxide (CO2)

EGR routes inert exhaust gas back into the intake to lower peak combustion temperature, reducing the formation of NOx. A clogged EGR passage can cause knock and high NOx, while a stuck-open valve causes rough idle. Its operation is monitored by OBD-II.

A no-start with no spark and no injector pulse, but a good battery and cranking, most likely points to:

• a.A worn water pump
• b.A clogged cabin filter
• c.A failed crankshaft position sensor or PCM/power issue✓
• d.A leaking A/C condenser

When both spark and injector pulse are absent during cranking, a shared input such as the crankshaft position sensor or a PCM power/ground problem is a prime suspect. Scan-tool rpm during cranking helps confirm the crank signal. Systematic checks of power, ground, and reference sensors isolate the cause.