Diagnostic guides by symptom

Structured logic walkthroughs for the complaints techs actually see in the bay. Each guide works the problem the way a diagnostician thinks — inputs, commands, expected results, and the thinking that separates a fix from a guess.

• No-Crank No-Start: Diagnose It With Logic, Not Guesses
  Stop swapping starters and batteries. Diagnose a no-crank no-start by working the circuit with real measurements and structured logic.
• Crank-No-Start: Spark, Fuel, Compression, Sync — Worked in Order
  The engine cranks but won't fire. Use structured diagnostic logic to isolate spark, fuel, compression, or sync — with the data PIDs that prove each.
• Random Misfire Only When Hot: Think Before You Swap Coils
  P0300 random misfire only at operating temp? Diagnose it with structured logic — fuel trim, misfire counters, and heat-sensitive failure modes — before swapping parts.
• Oil Light On, But Pressure Tests Fine: Don't Replace the Pump
  Dash says low oil pressure, mechanical gauge says fine. Use structured logic to isolate the sensor, wiring, or PCM logic before touching the pump.
• CAN Bus Diagnostics: From U-Codes to Root Cause
  Diagnose CAN bus failures with structured logic — termination, bias voltages, waveforms, and module isolation — before you start unplugging things at random.
• Voltage Drop Testing: The Test That Finds What Ohms Can't
  Voltage drop testing finds the resistance an ohmmeter misses. Use it as the foundation of every electrical diagnosis — with the thresholds and method that matter.
• Is It Really the Fuel Pump? Confirm Before You Drop the Tank
  Don't replace a fuel pump on a guess. Use pressure, volume, voltage, and current draw — structured logic — to confirm the pump is the actual failure.
• Why Fuel Pressure Alone Doesn't Condemn the Fuel Pump
  A pressure gauge in spec doesn't mean the pump is good — and out of spec doesn't mean it's bad. Here's the structured logic to actually condemn a pump.
• No-Crank After Starter Replacement: What You Missed the First Time
  Installed a new starter and it still won't crank? The starter was the symptom, not the cause. Work the circuit with structured logic before pulling it out again.
• When the RPM PID Lies to You
  The RPM PID is the most-trusted PID on the scan tool — and one of the easiest to misread. Here's how to know when the number on the screen isn't telling the truth.
• Why Continuity Tests Fail Voltage Drop Problems
  An ohmmeter says the circuit is fine. Under load, it fails. Here's why continuity testing misses what voltage drop catches — and how to stop being fooled.
• Why the Code Isn't the Diagnosis
  A DTC tells you what the PCM noticed — not what's broken. Here's how to use codes as a starting point instead of a parts list.

More symptoms being added. Working a vehicle that's not listed? Start a guided diagnostic →