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WorksheetsOxygen sensors and fuel trim quiz
Total questions: 20
Worksheet time: 10mins
Which statement best describes the primary role of oxygen sensors in gasoline engines?
Monitor exhaust oxygen to guide fuel delivery
Measure intake air temperature for spark timing
Regulate coolant flow to stabilize engine heat
Control throttle position to reduce idle vibration
What air-fuel ratio is considered ideal for most gasoline engines?
10.0 parts air to 1 part fuel
16.0 parts air to 1 part fuel
12.5 parts air to 1 part fuel
14.7 parts air to 1 part fuel
A narrowband zirconia O2 sensor typically outputs which voltage range?
0.1 to 0.9 volts under operation
1.5 to 3.0 volts at all times
5.0 volts fixed reference output
0.01 to 0.05 volts during idle
In titania O2 sensors, how is oxygen content conveyed to the PCM?
By a constant current regardless of mixture
By a mechanical diaphragm moving a switch
By a generated voltage varying with oxygen level
By resistance changes needing a reference voltage
Wideband (A/F or WRAF) sensors differ from narrowband sensors primarily because they
Send a current signal measuring exact ratio
Operate only downstream of the converter
Switch rapidly only near stoichiometric point
Require no electrical connection to the PCM
A vehicle shows 0.9 V from a narrowband sensor. Which conclusion is most justified for immediate fuel control?
Mixture is unknown; hold fuel delivery steady
Mixture is lean; increase fuel delivery slightly
Mixture is rich; reduce fuel delivery slightly
Mixture is perfect; no adjustment is needed
Where is the downstream O2 sensor typically located in the exhaust system?
Before the catalytic converter near the manifold
After the catalytic converter in the exhaust stream
Inside the intake manifold upstream of airflow
Beside the muffler outside the exhaust flow
Which statement best describes Short-Term Fuel Trim (STFT)?
Indicates injector pulse width programming only
Measures catalytic converter efficiency stability
Adjusts fuel immediately based on sensor feedback
Learns long-term corrections over many drives
A vehicle shows LTFT of +9% and STFT fluctuating around 0%. What is the most reasonable conclusion?
PCM is subtracting fuel due to rich condition
PCM is adding fuel to correct a persistent lean
Catalytic converter is failing and unstable
MAF sensor is always reading perfectly
Which scenario most likely leads to negative fuel trim values?
Vacuum leak introducing unmetered air
Dirty MAF sensor under-reporting airflow
Weak fuel pump causing low pressure
Leaking injectors creating rich mixture
Which issue most commonly causes a lean condition and positive fuel trim?
Excessive fuel pressure from regulator
Stuck-open purge valve enriching mix
Vacuum leaks introducing extra air
Leaking injectors increasing rail fuel
A vehicle shows negative fuel trim. Which cause best fits a rich condition?
Cracked intake boot adding air
Weak fuel pump reducing pressure
Exhaust leak before upstream sensor
Faulty coolant temp sensor reading cold
You suspect a dirty MAF sensor. What fuel trim behavior is most likely?
Positive trim due to underreported air
Negative trim due to extra fuel
Oscillating trim unrelated to airflow
No change because MAF is downstream
Which oxygen sensor primarily controls fuel trim in modern engines?
Upstream sensor before catalytic converter
Downstream sensor after catalytic converter
Wideband sensor inside catalytic converter
Temperature sensor near exhaust manifold
A positive fuel trim value is most likely indicating which condition?
Catalyst efficiency above specification
Lean mixture needing added fuel
Rich mixture needing less fuel
Perfect combustion at lambda 1.0
What is the ideal air-fuel ratio for gasoline engines under stoichiometric conditions?
12.5 parts air to 1 fuel
13.2 parts air to 1 fuel
14.7 parts air to 1 fuel
16.0 parts air to 1 fuel
Which statement best describes narrowband oxygen sensor output behavior?
Provides exact AFR measurement continuously
Outputs current in milliamps for AFR
Switches between rich and lean voltage
Reports catalyst efficiency downstream
On a wideband sensor, which reading suggests a lean mixture?
Higher lambda above 1.0 value
Lower lambda below 1.0 value
AFR around 14.7:1 value
Lambda exactly equals one value
During a catalyst monitor test, what pattern from the downstream O2 sensor indicates a failing catalytic converter?
Mirrors the upstream sensor pattern
Shows steady low-frequency changes
Stays fixed near stoichiometric value
Displays higher voltage at all times
Which fuel trim responds immediately to oxygen sensor feedback during driving?
Catalyst monitor trim (CMT)
Long-term fuel trim (LTFT)
Upstream monitor trim (UMT)
Short-term fuel trim (STFT)
