CURATED AUTOMOTIVE DIRECTORY
By Trouble Code
Understand what a diagnostic trouble code monitors, what it does not prove, and which tests come next.
ARTICLES & MATERIALS
Start with these guides
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P0300 Random/Multiple-Cylinder Misfire: Meaning and Tests
The controller detected crankshaft-speed variation consistent with combustion loss across more than one cylinder.
Read guideP0171 System Too Lean Bank 1: A Fuel-Trim Workflow
The controller reached a calibrated positive fuel correction because feedback indicated a lean condition on bank 1.
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P0420 Catalyst Efficiency Below Threshold: What to Prove First
The catalyst monitor judged oxygen-storage performance below its calibrated threshold; the code does not prove the converter is the first failed part.
Read guideP0455 EVAP Large Leak Detected: A Sealed-System Test Path
The evaporative monitor could not create or hold the expected vacuum or pressure, or it detected flow consistent with a large leak.
Read guideP0128 Coolant Temperature Below Thermostat Regulating Range
Warm-up behavior did not match the calibrated time and load model, commonly because measured coolant temperature rises too slowly.
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P0562 System Voltage Low: Battery, Charging, or Connection?
A control module measured supply voltage below its calibrated threshold for the required time.
Read guideP0135 Oxygen Sensor Heater Circuit: Test the Circuit
The controller detected an electrical or performance fault in the bank 1 sensor 1 heater circuit.
Read guideP0442 EVAP Small Leak Detected: Finding a Small Leak
The EVAP monitor detected leakage smaller than a large-leak threshold, which makes test setup and stable sealing especially important.
Read guideU0100 Lost Communication With ECM/PCM: Network Triage
Another module reported that expected communication with the engine or powertrain controller was absent.
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C0035 Left-Front Wheel-Speed Circuit: Signal Before Sensor
A chassis controller detected a left-front wheel-speed circuit or signal fault; code wording varies by manufacturer.
Read guideUse these guides to plan checks, not to replace model-specific service information. Stop when a procedure involves unsupported lifting, pressurized fuel, pyrotechnic restraints, refrigerant recovery, or high-voltage isolation beyond your training and equipment.