Most Common OBD2 Error Codes Explained
- Mark Tse
- May 18
- 4 min read
Your check engine light is on and you've got a code. Now what? Here's a plain-English breakdown of the most common OBD2 codes, what they mean, and what you should do about each one.
P0171 — System Too Lean (Bank 1)
What it means: Your engine is getting too much air or not enough fuel. The air-to-fuel ratio is off on the left side (Bank 1) of the engine.
Common causes:
Vacuum leak (cracked hose or loose intake manifold)
Dirty or failing mass airflow (MAF) sensor
Clogged fuel injectors
Weak fuel pump
Faulty oxygen sensor
How serious is it? Moderate. Driving long-term with a lean condition can damage your engine. Get it diagnosed within a week.
Typical repair cost: $50–$500 depending on the cause. A vacuum leak fix can be very cheap; a fuel pump replacement is more expensive.
P0300 — Random/Multiple Cylinder Misfire Detected
What it means: Your engine is misfiring, but the misfire isn't isolated to one cylinder. Multiple cylinders are affected.
Common causes:
Worn or fouled spark plugs
Bad ignition coils
Clogged fuel injectors
Low fuel pressure
Vacuum leak
Low compression (more serious)
How serious is it? High. A misfire can damage your catalytic converter quickly and indicates your engine isn't running efficiently. If the check engine light is flashing, pull over immediately.
Typical repair cost: $100–$400 for spark plugs and coils. Low compression issues can cost $1,000+.
P0301–P0308 — Cylinder-Specific Misfire
What it means: Same as P0300, but the number tells you exactly which cylinder is misfiring. P0301 = cylinder 1, P0302 = cylinder 2, and so on.
Why this is useful: A cylinder-specific misfire makes diagnosis much easier. You can swap that cylinder's ignition coil with an adjacent one and see if the misfire follows the coil — if it does, you've found your problem.
Common causes: Ignition coil, spark plug, or fuel injector specific to that cylinder.
Typical repair cost: $50–$300 per coil or plug.
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
What it means: Your catalytic converter isn't working as efficiently as it should. It's not converting harmful exhaust gases well enough.
Common causes:
Worn-out catalytic converter
Faulty oxygen sensor giving false readings
Engine oil burning (fouling the cat)
Coolant leak into exhaust
How serious is it? Moderate to high. Your car will eventually fail emissions tests. The catalytic converter itself is expensive to replace, but rule out a bad O2 sensor first — it's much cheaper.
Typical repair cost: $150–$300 for an O2 sensor. $800–$2,500 for a catalytic converter replacement.
P0442 — Evaporative Emission Control System Leak Detected (Small Leak)
What it means: There's a small leak in your evaporative emissions system — the system that prevents fuel vapors from escaping into the atmosphere.
Common causes:
Loose or faulty gas cap (most common)
Cracked or disconnected EVAP hose
Faulty purge valve or vent valve
How serious is it? Low. This won't affect your car's performance or drivability. But you'll fail emissions tests and the light will stay on.
What to try first: Remove your gas cap, inspect the rubber seal, and reinstall it firmly. If the light clears after a few days, that was your problem.
Typical repair cost: $5 for a new gas cap to $200 for a purge valve.
P0455 — Evaporative Emission Control System Leak Detected (Large Leak)
What it means: Same as P0442, but a larger leak in the EVAP system.
Common causes:
Missing or severely damaged gas cap
Large crack in EVAP hose
Failed charcoal canister
How serious is it? Low for drivability, but you'll need it fixed for emissions. A large EVAP leak can sometimes indicate a more significant problem than a loose gas cap.
Typical repair cost: $20–$600 depending on what's leaking.
P0505 — Idle Control System Malfunction
What it means: Your car's idle speed control system isn't functioning properly. Your engine may idle too high, too low, or roughly.
Common causes:
Dirty or faulty idle air control (IAC) valve
Vacuum leak
Throttle body buildup
How serious is it? Moderate. A rough or unstable idle is uncomfortable and can eventually cause stalling.
Typical repair cost: $50–$300. Cleaning the IAC valve or throttle body is often a cheap fix.
P0128 — Coolant Temperature Below Thermostat Regulating Temperature
What it means: Your engine isn't reaching its normal operating temperature. The thermostat is likely stuck open, allowing too much coolant flow.
Common causes:
Failing or stuck-open thermostat (most common)
Faulty coolant temperature sensor
How serious is it? Moderate. Running too cold reduces fuel economy, increases engine wear, and affects heater performance. Thermostats are inexpensive to replace.
Typical repair cost: $150–$300 for a thermostat replacement.
P0011 / P0021 — Camshaft Position Timing Over-Advanced
What it means: Your engine's variable valve timing (VVT) system has the camshaft timing further advanced than it should be.
Common causes:
Low or dirty engine oil (most common — change your oil first)
Faulty camshaft phaser
Failed VVT solenoid
Stretched timing chain
How serious is it? High. Timing issues can cause significant engine damage. If you're overdue for an oil change, do that first — it's the cause more often than people expect.
Typical repair cost: $50 for an oil change to $1,000+ for a phaser or solenoid replacement.
How to Handle Any OBD2 Code
Whatever code you're dealing with, the process is the same:
Read the code with an OBD2 scanner
Understand what system it relates to and how serious it is
Check for the cheapest possible cause first (loose gas cap, low oil, dirty sensor)
If it's beyond a simple fix, get a mechanic's estimate — but go in knowing what the code is so you can't be oversold
Ratchet AI reads your OBD2 codes and explains them in plain English through voice — so you know exactly what you're dealing with before you ever set foot in a shop.
Get Ratchet AI at tryratchet.com
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