Fault diagnosis
A fault code is a pointer, not a finding. "Oxygen sensor signal implausible" does not mean the sensor is dead — it means the control unit is receiving a signal that does not match the rest. The cause may be an air leak, a broken wire, or indeed the sensor. Read the code and swap the part, and you often swap twice.
What it covers
- Reading all control units, not only the engine ECU
- Looking at the freeze-frame data: revs, load and temperature when it occurred
- Following live values under operation instead of only reading the stored code
- Clearing, driving, reading again — otherwise it is unclear whether the fault is still live
When it makes sense
- A warning light stays on or keeps coming back
- The car runs in limp mode with reduced power
- Before buying a used car, as a look into stored and cleared faults
- Fuel consumption that has risen for no visible reason
Worth knowing
Before buying used, a diagnostic readout is the cheapest safeguard there is. What matters is less what is stored than what is missing: a car with a completely empty fault memory and high running hours was cleared shortly before the viewing. An honest car has a few harmless entries.
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