Engine Performance & Diagnostics Flashcards

A wideband air/fuel ratio sensor on a scan tool reads 0.85 lambda at steady cruise. What does this indicate?

Answer: Rich mixture

Lambda below 1.0 means less air than stoichiometric, which is a rich mixture.

An upstream narrowband O2 sensor is stuck at about 0.1 volts while the engine runs. Which is the most likely cause?

Answer: Exhaust leak ahead of the sensor or a lean condition

A low, steady narrowband voltage indicates oxygen-rich exhaust caused by a lean mixture or outside air drawn in through an exhaust leak.

A P0420 catalyst efficiency code is set. The downstream O2 sensor's switching pattern closely matches the upstream sensor. What does this mean?

Answer: Catalyst has lost oxygen storage capacity

A healthy catalyst stores oxygen and keeps the downstream signal steady, so a mirrored signal shows lost storage capacity.

Which MAF sensor reading at warm idle on a typical 2.0L engine indicates a likely contaminated or under-reporting sensor?

Answer: 1.0 g/s

The rule of thumb is about 1 g/s per liter at idle, so 1.0 g/s on a 2.0L engine is about half the expected value.

A technician suspects a skewed engine coolant temperature sensor. The engine is fully warm, but the scan tool reads 40°F (4°C). What driveability symptom is most likely?

Answer: Rich running and poor fuel economy

The PCM thinks the engine is cold and adds enrichment, causing rich running and poor fuel economy.

Which scan tool PID best confirms that the engine has entered closed-loop operation?

Answer: Fuel system status

The fuel system status PID shows directly whether the PCM is in open loop or closed loop.

Freeze frame data for a misfire code shows the fault occurred at 1,800 RPM, 75% load, and 35°F intake temperature. Why is this data valuable?

Answer: It lets the technician recreate the conditions during a road test

Freeze frame records the operating conditions when the fault set, so the technician can reproduce the failure.