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Level 4 · Diagnostics OBD-II Lesson 2/2 intermediate 10 min

Live Data and Readiness

Turning a list of numbers into a diagnosis.

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What you will learn

  • Choose the PIDs that matter for a symptom
  • Compare live data against expected behaviour
  • Use readiness monitors to confirm a repair
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Live data is a stream of parameters. The skill is choosing the right ones and watching how they move together. For a lean code you want fuel trims, MAF or MAP, coolant temperature and O2 response. For a misfire you want misfire counters, ignition timing and fuel trim per bank.

Readiness monitors tell you whether the ECU has completed its self-tests. Clearing codes resets them, so a car can pass a scan without being genuinely repaired.

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CASE 004 P0171 System Too Lean (Bank 1) Open case →

Rough idle, occasional hesitation and a check engine light. Fuel consumption has increased slightly.

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Key points

  • More data is not more information — select what matters
  • Look for relationships, not isolated numbers
  • Readiness proves the ECU has genuinely tested the system
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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