These are the ten OBD2 codes our customers ask about most. For each code: what it literally means, the most common root causes (in order of frequency), and whether you can DIY the fix. Bookmark this — the check-engine light will come on eventually.
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
What it means: The rear O2 sensor isn't seeing the emissions drop it expects after the catalytic converter. Either the converter is degraded, or the sensor is reading wrong.
Most common causes, in order:
- Aging catalytic converter (80%+ of the time on vehicles over 100k miles)
- Downstream O2 sensor failure (~10%)
- Exhaust leak before the rear O2 sensor (~5%)
- Bad upstream fuel trim pushing too-rich exhaust into the cat (~5%)
DIY or shop? Read fuel trim + O2 sensor live data before replacing anything. A new cat is $400–$2,000; a new rear O2 sensor is $60. Order matters.
P0171 — System Too Lean (Bank 1)
What it means: The engine is running with too much air relative to fuel. The ECM added fuel to compensate but ran out of room.
Most common causes:
- Vacuum leak (cracked intake boot, PCV hose, brake booster line)
- Failing mass-airflow sensor (MAF)
- Weak fuel pump or clogged filter
- Dirty or dead fuel injector on a single cylinder
Smoke test the intake first — a shop can do it for $50. Smoke will leak out of any vacuum crack. Very common on 2006–2014 BMW N54 and N55 engines (charge pipe).
P0300 — Random/Multiple Cylinder Misfire Detected
What it means: The engine is misfiring and the ECM can't pin it to a single cylinder.
Most common causes:
- Failing ignition coil(s) — very common on VAG and BMW
- Worn spark plugs (overdue interval)
- Low fuel pressure
- Vacuum leak affecting multiple cylinders
If you also see P0301–P0308 (specific cylinder misfire codes), that's your cylinder. Swap the coil from a misfiring cylinder to a healthy one and re-scan — if the misfire moves with the coil, it's the coil.
P0455 — Evaporative Emission System Leak Detected (Large Leak)
What it means: The EVAP system isn't holding pressure. Fuel vapors are escaping.
Most common cause: Loose, cracked, or missing gas cap. Seriously — it's the first thing to check. If tightening doesn't fix it after 50 miles of driving, the next culprits are the purge valve, vent solenoid, or a cracked EVAP hose.
P0128 — Coolant Thermostat (Coolant Temp Below Regulating Temperature)
What it means: The engine isn't reaching operating temp fast enough. Almost always a stuck-open thermostat.
The DIY fix: Replace the thermostat ($25–$80 part) and refill coolant. Two-hour job on most vehicles. Worth doing — a stuck-open thermostat means the ECM never leaves open-loop fueling, which wrecks fuel economy.
P0401 — EGR Insufficient Flow Detected
What it means: The exhaust gas recirculation system isn't flowing enough.
Most common cause: Carbon buildup in the EGR valve or passages. On diesels (including heavy-duty trucks scanned with our UCAN-HD-A), this is the single most common code we see past 150k miles.
P0442 — Evaporative Emission System Leak Detected (Small Leak)
What it means: Same system as P0455, smaller leak. Often a gas-cap seal, sometimes a pinhole in an EVAP hose.
DIY test: Replace the gas cap first ($10–$25). If the code returns after 100 miles, a shop can smoke-test the EVAP system.
P0301–P0308 — Cylinder #N Misfire Detected
The last digit is the cylinder number. P0301 = cylinder 1. P0304 = cylinder 4. Troubleshoot exactly like P0300, but you already know which cylinder to start with. Swap coils or spark plugs with an adjacent cylinder to confirm.
P0011 — Camshaft Position Timing Over-Advanced (Bank 1)
What it means: Variable valve timing (VVT) isn't working correctly. The cam is too far advanced or the oil-control solenoid is stuck.
Most common cause on higher-mileage engines: Clogged VVT solenoid screen (a $20 part and 30-minute DIY on many engines). Before replacing timing components, clean the solenoid.
U0100 — Lost Communication with ECM/PCM
What it means: Something on the CAN bus lost contact with the engine control module. This is the single most-serious code on this list — it usually means an actual wiring or module failure, not a sensor issue.
Do not ignore this one. A bi-directional scanner (our UCAN-II-C Pro does this) can ping each module to find which one dropped off the bus. Without that capability, you're guessing.
Related reading
- How to read and clear OBD2 codes — the complete beginner's workflow.
- Bi-directional scanner explained — when a $40 code reader isn't enough.
Tools we recommend
- UCAN-II-C Pro — reads all the codes above plus manufacturer-specific codes, lifetime updates. $459.
- The full YOUCANIC shop — scanners, battery testers, jump starters, borescopes.
References
- SAE J2012 — Diagnostic Trouble Code Definitions (full reference) — SAE International (accessed 2026-04-22)
- Be Car Care Aware — Common Check Engine Light Causes — Car Care Council (accessed 2026-04-22)
- NHTSA — Vehicle Recall and Service Bulletin Database — US National Highway Traffic Safety Administration (accessed 2026-04-22)
- EPA — Catalytic Converter Requirements and OBD Monitoring — US Environmental Protection Agency (accessed 2026-04-22)
Discover more from YOUCANIC or browse the full YOUCANIC collection.
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