Whether you’re a seasoned mechanic or a hands-on DIY vehicle owner, understanding vehicle fault codes, also known as diagnostic trouble codes (DTCs) or OBD2 codes, is essential for accurate and efficient repairs. Modern vehicles rely heavily on electronic systems, and these codes act as the first clue to uncovering hidden issues.

In this guide, we break down the 10 most common fault codes, explain what they mean, and show you how to diagnose them like a pro. By the end, you'll feel more confident tackling repairs and using your diagnostic tools to their full potential.

Top 10 Most Common Vehicle Fault Codes

1. P0300 - Random/Multiple Cylinder Misfire Detected

A misfire occurs when fuel in one or more cylinders fails to ignite properly.

Common Causes: Spark plugs, ignition coils, vacuum leaks, clogged injectors, low compression.

How to Pinpoint the Exact Fault

• Check misfire counters in live data on a scan tool to isolate the affected cylinder/cylinders

• Swap ignition coils/plugs between cylinders - if the misfire moves, you’ve found the fault.

• Perform a smoke test to rule out intake/vacuum leaks.

• Run a balance test or injector disable test (if your scan tool supports it).

• Perform a compression test if misfires remain after ignition checks.

2. P0171 - System Too Lean (Bank 1)

The engine is receiving too much air / too little fuel.

Common Causes: Intake leaks, MAF faults, weak fuel pump, O2 sensor issues.

How to Pinpoint the Exact Fault

• Check fuel trims:

  • STFT + LTFT above +15% confirms a lean condition.

• Raise RPM to 2500:

  • If trims normalize → intake/vacuum leak.

  • If trims worsen → fuel delivery issue.

• Smoke test the intake (the fastest way to find leaks).

• Check MAF g/s readings for your engine size (rough rule: ~1 g/s per litre at idle).

• Block off PCV lines temporarily to see if trims drop.

• Measure fuel pressure under load.

3. P0420 - Catalyst System Efficiency Below Threshold

This code indicates possible catalytic converter performance issues.

Common Causes: Catalytic converter failure, O2 sensor problems, exhaust leaks.

How to Pinpoint the Exact Fault

• Check for exhaust leaks first (before spending $$).

• Graph upstream (sensor 1) and downstream (sensor 2) O2 sensors:

  • Upstream = should switch rapidly.

  • Downstream = should stay stable.

  • If downstream mirrors upstream → bad cat.

• Perform a 3000 RPM hold test and watch sensor behaviour.

• Check fuel trims - Rich/lean issues can damage cats.

4. P0442 - EVAP System Leak (Small Leak)

Common Causes: Loose/old gas cap, cracked hoses, stuck vent valve.

How to Pinpoint the Exact Fault

• Inspect gas cap O-ring for cracks or flattening.

• Perform an EVAP smoke test at the purge valve.

• Use bi-directional tests using a scan tool:

  • Command vent valve closed

  • Command purge valve open 

  • Look for pressure drop in tank pressure sensor.

• If pressure doesn’t hold → locate leak with smoke.

5. P0455 - EVAP System Leak (Large Leak)

Common Causes: Disconnected EVAP hose, faulty purge/vent valve.

How to Pinpoint the Exact Fault

• Run a full EVAP smoke test while sealing the system.

• Check vent valve operation - stuck open vents cause P0455.

• Inspect charcoal canister for cracks (often overlooked).

• Use pressure test through tank filler neck if smoke isn’t effective.

6. P0113 - Intake Air Temperature Sensor Circuit High Input

Common Causes: Open circuit, faulty IAT, unplugged sensor, damaged wiring.

How to Pinpoint the Exact Fault 

• Check live data: If IAT reads –40°C or extremely high → circuit issue.

• Inspect connector pins for corrosion or spread terminals.

• Wiggle test the harness to see if the reading fluctuates.

• Backprobe the IAT:

  • 5V reference present?

  • Good ground?

• If both good → Replace sensor.

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7. P0128 - Coolant Thermostat Below Regulating Temperature 

Common Causes: Stuck-open thermostat, low coolant, faulty ECT sensor.

How to Pinpoint the Exact Fault

• Monitor coolant warm-up:

  • Should reach operating temp in a set time.

• Compare ECT reading with infrared thermometer at thermostat housing.

• Check coolant level & air pockets.

• If temps differ significantly → ECT sensor issue.

8. P0500 - Vehicle Speed Sensor Malfunction

Common Causes: Damaged VSS, broken tone ring, wiring faults.

How to Pinpoint the Exact Fault

• Check live speed data from the VSS.

• If speed reads “0” while driving → sensor or wiring.

• Inspect VSS connector and wiring where it enters the gearbox.

• Scope the sensor (if available) to confirm signal output. 

• Check ABS speed inputs - some vehicles use ABS data instead of a physical VSS.

9. P0133 — O2 Sensor Circuit Slow Response

Common Causes: Aged O2 sensor, exhaust leaks, contamination.

How to Pinpoint the Exact Fault

• Monitor sensor switching:

  • Should switch 0.1–0.9V rapidly at idle.

• Perform snap-throttle test:

  • Rich spike → lean dip should be immediate.

• Check for exhaust leaks BEFORE sensor 1 (common cause).

• Compare both banks — if only one is slow → failing sensor.

10. P0700 - Transmission Control System Malfunction

This code simply means the TCM has stored its own codes.

Common Causes: Internal transmission faults, solenoid issues, TCM communication errors.

How to Pinpoint the Exact Fault

• Use a full-system scan tool capable of reading TCM data (cheap readers cannot).

• Retrieve transmission-specific codes (P07xx range).

• Check TCM live data:

  • Gear command vs. actual

  • Shift solenoid performance

  • Line pressure

• Inspect harness to TCM for water intrusion or corrosion.

• If codes relate to slip → may require pressure testing or internal inspection.

Additional Tips for Diagnosing Vehicle Fault Codes

• Always check freeze-frame data: This snapshot reveals the exact engine conditions when the code was triggered.

• Don’t clear codes too early: Doing so removes valuable diagnostic data.

• Look for patterns: Repeat codes often point to underlying system failures.

• Use professional-grade tools: Cheap readers often miss manufacturer-specific codes.

• Avoid parts-swapping: Data-based diagnostics saves time and money.

Understanding and properly analyzing OBD2 codes are essential skills for modern repair work. With the right tools and techniques, diagnosing these common issues becomes faster, more accurate, and far more profitable for workshops.

Explore our professional-grade tools at Premium Diagnostic Equipment and elevate your workshop efficiency today.