Top 100 ECU Fault Codes Explained

If your check engine light just popped on and you’re staring at a code like P0420 or P0300 wondering what the hell it means… you’re in the right place.

I’ve been wrenching on cars and tuning ECUs for over 20 years. Street cars, track cars, daily drivers, you name it. I’ve seen thousands of these codes. Some are nothing. Some will leave you stranded.

Today I’m breaking down the Top 100 ECU Fault Codes in plain English. No textbook talk. Just real talk from the garage.

You’ll get:

What the code actually means
What usually causes it
How serious it is
What you can try yourself before throwing parts at it

Let’s get your car sorted.

How ECU Fault Codes Work (Quick & Simple)

Your car’s ECU (Engine Control Unit) is basically the brain. It watches dozens of sensors. When something’s off, it throws a code and turns on the check engine light.

Codes look like this:

P0xxx – Generic powertrain (most common)
P1xxx – Manufacturer specific
P2xxx – Generic more recent
C0xxx – Chassis
B0xxx – Body
U0xxx – Network/communication

We focus on the P codes because those are what 95% of you are dealing with.

Always start with a good OBD2 scanner. Don’t guess. Read the code, write it down, then clear it and see if it comes back.

I grouped the most common ones you’ll actually see. I put the absolute most frequent ones first with extra detail, then a big clean list of the rest.

The Real Heavy Hitters (These Come Up Constantly)

1. P0300 – Random/Multiple Cylinder Misfire

Engine is misfiring but not on one specific cylinder.

Common causes: Bad spark plugs, weak coils, dirty injectors, vacuum leak, low fuel pressure, or even a bad tank of gas.

Feels like: Rough idle, loss of power, shaking.

Fix tip: Start with plugs and wires/coils. Check for vacuum leaks with carb cleaner spray.

2. P0301 to P0308 – Cylinder Specific Misfire

Same as above but points to exact cylinder (P0301 = cylinder 1).

Usually coil pack or plug on that cylinder. Swap coil to another cylinder — if the code follows, you found it.

3. P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

Your catalytic converter isn’t cleaning the exhaust like it should.

Common causes: Actually dying cat, bad downstream O2 sensor, exhaust leak, or engine running rich/lean for a long time.

Don’t just throw a cat at it. Check the O2 sensors and fix any running issues first.

4. P0430 – Same as P0420 but Bank 2

V6 and V8 engines.

5. P0171 – System Too Lean (Bank 1)

Too much air or not enough fuel.

Big causes: Vacuum leaks (very common), dirty MAF sensor, weak fuel pump, clogged injectors.

Spray brake cleaner around intake boots while idling — if RPM changes, you found the leak.

6. P0174 – System Too Lean (Bank 2)

7. P0172 – System Too Rich (Bank 1)

Opposite problem. Too much fuel.

Dirty MAF, leaking fuel pressure regulator, bad O2 sensor, or stuck injectors.

8. P0128 – Coolant Thermostat Below Regulating Temperature

Thermostat stuck open. Engine never reaches proper temp.

Easy fix on most cars. Cheap part.

9. P0455 – EVAP System Large Leak

Usually the gas cap. Seriously. Tighten it or replace it first.

Also cracked EVAP lines or bad purge valve.

10. P0456 – EVAP Small Leak

Same system, smaller leak. Harder to find. Smoke test is your friend.

11. P0113 – Intake Air Temperature Sensor High

Bad IAT sensor or wiring.

12. P0118 – Engine Coolant Temperature Sensor High

13. P0122 / P0123 – Throttle Position Sensor Low/High

14. P0130 to P0167 – All the Oxygen Sensor Codes

Upstream (pre-cat) and downstream.

P0135 / P0141 / P0155 / P0161 are heater circuit codes — very common.

15. P0401 – EGR Flow Insufficient

Carbon buildup in EGR valve or passages. Clean it.

Next Most Common (You’ll See These Weekly in Any Shop)

P0442 – EVAP small leak (different from 0456)
P0506 – Idle control lower than expected
P0507 – Idle higher than expected
P0700 – Transmission Control System Malfunction (just a flag — check for other trans codes)
P0741 – Torque Converter Clutch circuit
P0170 – Fuel Trim Malfunction
P0325 – Knock Sensor circuit
P0335 – Crankshaft Position Sensor
P0340 – Camshaft Position Sensor
P0351 to P0358 – Ignition Coil Primary/Secondary codes
P0400 – EGR Flow Malfunction
P0410 – Secondary Air Injection
P0440 – EVAP general
P0441 – EVAP incorrect flow
P0446 – EVAP vent control
P0500 – Vehicle Speed Sensor
P0601 to P0606 – ECU internal memory/processor errors (scary but sometimes just low voltage)
P0705 – Transmission Range Sensor
P0715 – Input/Turbine Speed Sensor
P0720 – Output Speed Sensor
P0730 – Incorrect Gear Ratio
P0841 to P0871 – Transmission fluid pressure sensor codes
P1128 / P1129 – Closed loop fueling not reached (manufacturer)
P1130 to P1138 – Various AF/O2 sensor codes (Toyota etc.)
P1150 to P1155 – More AF sensor stuff

Fuel, Air & Sensor Codes You’ll Meet

P0100 to P0105 – MAF/MAP sensor codes
P0101 – MAF range/performance
P0102 – MAF low input
P0106 to P0108 – MAP sensor
P0110 to P0112 – IAT sensor
P0115 to P0117 – ECT sensor
P0120 to P0124 – TPS codes
P0131 to P0134 – O2 sensor circuit bank 1 sensor 1
P0136 to P0139 – O2 bank 1 sensor 2
P0151 to P0154 – O2 bank 2 sensor 1
P0200 to P0208 – Injector circuit codes
P0217 – Engine overtemp
P0218 – Transmission overtemp
P0220 to P0229 – Throttle/Pedal sensor
P0230 to P0232 – Fuel pump circuit
P0299 – Turbo underboost (super common on modern turbos)
P0300 already covered
P0315 – Crankshaft variation not learned
P0320 to P0324 – Ignition/distributor engine speed
P0330 – Knock sensor bank 2

Cam, Crank, VVT & Timing Codes

P0341 to P0344 – Cam sensor performance
P0345 to P0349 – Cam sensor bank 2
P0365 to P0369 – Cam sensor exhaust
P0010 to P0011 – Camshaft position actuator (VVT)
P0012 – Cam timing over-retarded
P0013 to P0017 – More VVT and correlation codes
P0016 – Cam/Crank correlation bank 1
P0017 – Cam/Crank correlation bank 1 exhaust
P0018 / P0019 – Bank 2 correlation
P0020 to P0025 – Bank 2 VVT

Exhaust, Cat & EVAP Deep Cuts

P0421 – Warm up catalyst efficiency
P0431 – Warm up cat bank 2
P0443 – EVAP purge control valve
P0449 – EVAP vent valve/solenoid
P0451 to P0454 – EVAP pressure sensor
P0457 – EVAP leak (gas cap loose)
P0460 to P0463 – Fuel level sensor
P0480 to P0483 – Cooling fan codes
P0496 – EVAP high purge flow
P050A to P050B – Cold start idle codes

Transmission & Drivetrain Regulars

P0701 to P0703 – Trans control range
P0710 to P0714 – Trans fluid temp
P0716 to P0717 – Turbine speed
P0725 – Engine speed input
P0731 to P0736 – Gear ratio codes specific gears
P0740 – Torque converter clutch
P0750 to P0770 – Shift solenoid codes
P0776 / P0777 – Pressure control solenoid
P0790 to P0797 – More pressure and intermediate shaft
P0810 to P0880 – Various clutch and range switches

Quick Real-World Advice From the Garage

Battery voltage matters more than people think. Weak battery or bad ground = ghost codes.
Clear the code, drive it properly (mixed city/highway), then recheck.
Freeze frame data is gold. It shows exactly when the code set (rpm, load, temp).
On European cars and newer domestics, one code often brings friends. Fix the root cause.
If you’re tuning (Stage 1, 2, etc.), some codes are normal until you get the tune dialed. Others mean you went too far.

Tools I Actually Recommend

A decent Bluetooth OBD2 scanner + Torque Pro or OBD Fusion app will save you hundreds. For deeper stuff, a real bidirectional scan tool pays for itself.

There you go. The top 100 ECU fault codes explained like a buddy who actually works on cars every day would tell you.

Bookmark this page. Next time the light comes on, you’ll know exactly what’s up instead of panicking or getting robbed at a shop.

Now go fix that car and enjoy the drive. You’ve got this.

Any specific code giving you hell right now? Drop it in the comments — I read them all and try to help.

Drive safe,