A scan tool shows long-term fuel trim of +18%. What is the computer doing?
- Removing fuel to correct a rich condition
- Advancing spark to correct a lean condition
- Retarding spark to correct a rich condition
- Adding fuel to correct a lean condition
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D. Adding fuel to correct a lean condition
Positive trim means the computer has lengthened injector pulse because the oxygen sensor reported lean. Negative trim means it is taking fuel away. Fuel trim is about fuel delivery, not spark timing.
Reference: Fuel control; short-term and long-term fuel trim
A conventional zirconia oxygen sensor reads a steady 0.85 volt. What does this indicate about the exhaust?
- It is lean, with excess oxygen
- It is rich, with little oxygen
- It is at the ideal mixture
- It is too cool to be measured
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B. It is rich, with little oxygen
A zirconia sensor produces a high voltage (near 0.9 V) when there is little oxygen in the exhaust, meaning rich, and a low voltage (near 0.1 V) when lean. In normal closed loop it switches back and forth across about 0.45 V.
Reference: Oxygen sensor signal interpretation
What is freeze frame data?
- A record of engine conditions when a code set
- A list of every code the vehicle has ever set
- A record of when each monitor last ran
- A list of the sensors fitted to the engine
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A. A record of engine conditions when a code set
When an emission-related code sets, the computer stores a snapshot of data such as rpm, load, coolant temperature and fuel trim. It helps the technician recreate the conditions under which the fault occurred.
Reference: OBD-II freeze frame data