Original articles to understand the most common trouble codes: what they mean, how they show up, what causes them, and how they are fixed.
Basics
1. What is an OBD2 code and how to read it?
OBD-II (On-Board Diagnostics) is the self-diagnostic system that cars have had since the late 1990s. When the control unit detects a problem, it turns on the check engine light and stores a diagnostic trouble code (DTC). That code is the main clue to what is wrong with the vehicle.
An OBD2 code has a letter and four hexadecimal characters. The letter indicates the system:
P — Powertrain (engine and transmission). The most common.
C — Chassis (ABS, suspension, steering).
B — Body (airbags, locks, power windows).
U — Network and communication between modules.
To read it you need an OBD2 scanner (from $15–30 for the basic Bluetooth ones that work with your phone). It plugs into the OBD port, usually under the dashboard on the driver's side. The scanner shows the code and often lets you clear it.
What to do when the light comes on
Write down the exact code (e.g. P0420, not just "catalytic converter").
Don't clear the code without a reason: clearing it also erases the freeze-frame data that helps with the diagnosis.
Look up the code on CodeFinder to understand what it means and watch a solution video.
If the fault affects the brakes, steering, or airbags, don't drive and go to a workshop.
P0420 indicates that the bank 1 catalytic converter (the cylinder 1 side) is not filtering the exhaust gases with the expected efficiency. It is one of the most frequent codes and, at the same time, one of the most confusing, because the cause is not always the catalytic converter itself.
Symptoms
Check engine light on (sometimes with no other symptoms).
Slight increase in fuel consumption.
Rotten-egg smell from the exhaust in some cases.
Loss of power or rough idle if there is a significant leak.
Common causes
Worn or damaged catalytic converter.
Faulty oxygen (O2) sensor giving a false reading.
Exhaust leak before or near the catalytic converter.
Misfires that damage the catalytic converter over time.
How to diagnose it
The first step is to rule out exhaust leaks and check the condition of the oxygen sensors with a scanner that reads live data. A technician compares the signal of the oxygen sensor before and after the catalytic converter: if both oscillate the same, the catalytic converter is not doing its job.
Solution
Repair any leak or faulty sensor first, before replacing the catalytic converter, which is the expensive part. If the catalytic converter is truly worn out, the only definitive solution is to replace it.
P0300 appears when the control unit detects misfires in several cylinders or at random. It is an important code because a sustained misfire can damage the catalytic converter and increase fuel consumption and emissions.
Symptoms
Engine shaking or vibration at idle.
Jerking or loss of power when accelerating.
Steady or flashing check engine light (flashing indicates a risk to the catalytic converter).
Higher fuel consumption.
Common causes
Worn spark plugs or ignition coils.
Dirty or faulty injectors.
Vacuum or intake leaks.
Low compression from mechanical wear.
How to fix it
Start with the cheap stuff: inspect and replace the spark plugs and coils if they are due by mileage. Then check for vacuum leaks (hoses, intake gasket) and the condition of the fuel. If the fault persists, a compression test is worth doing to rule out an internal mechanical problem.
P0171 means the air-fuel mixture is "lean": too much air or too little fuel in bank 1 of the engine. The control unit tries to compensate by adding fuel, but if the correction exceeds a limit, it stores the code.
Symptoms
Rough or falling idle.
Lack of power or hesitation when accelerating.
Check engine light on.
Common causes
Vacuum or intake leaks (the most typical cause).
Dirty or faulty MAF sensor.
Low fuel pressure (pump or filter).
Faulty oxygen sensor.
How to fix it
Look for vacuum leaks first: a smoke test or sprayed starting fluid helps locate them. Clean or replace the MAF sensor and check the fuel pressure. An orderly diagnosis avoids replacing parts blindly.
The EVAP system prevents fuel vapors from the tank from escaping into the atmosphere. P0442 indicates a small leak in that circuit. It is almost always a simple and cheap problem to fix.
Symptoms
Usually only the check engine light, with no driving symptoms.
Sometimes a slight gasoline smell.
Common causes
Loose or faulty gas cap (the number one cause).
Cracked EVAP hose.
Faulty purge or vent valve.
How to fix it
Check the gas cap first: tighten it properly or replace it if the seal is dried out. If the code returns, check the hoses and valves of the EVAP circuit. It is a fault that rarely affects driving, but it is worth fixing to pass the emissions inspection.
6. Code P0128 — Coolant temperature below thermostat threshold
P0128 appears when the engine takes too long to reach operating temperature or fails to maintain it. The most common cause is a thermostat stuck open, which makes the coolant circulate constantly and keeps the engine from warming up.
Symptoms
The temperature gauge doesn't reach the middle zone.
The heater takes a long time to blow warm air (or blows little).
Higher fuel consumption.
Common causes
Thermostat stuck in the open position.
Low coolant level.
Faulty coolant temperature sensor (ECT).
How to fix it
The typical repair is to replace the thermostat, an inexpensive part. First, check the coolant level and rule out a faulty sensor. Driving with a cold engine for a long time increases wear and fuel consumption.
7. Code P0335 — Crankshaft position sensor circuit
The crankshaft position sensor (CKP) tells the control unit where the crankshaft is, so it can calculate ignition and injection timing. If it fails, the engine may not start or may stall while driving.
Symptoms
The engine won't start or starts with difficulty.
The engine stalls or shuts off while driving.
Jerking or misfires.
Common causes
Faulty CKP sensor.
Damaged wiring or connector.
Damaged tone ring.
How to fix it
Check the sensor connector and wiring before replacing it. If the sensor is faulty, replacing it is usually straightforward and moderately priced. Don't ignore it: a CKP failure can leave you stranded.
P0700 is a "generic" code that only indicates that the transmission control module (TCM) has detected a fault and requested that the check engine light be turned on. The real cause lies in transmission-specific codes that a scanner capable of reading the TCM can display.
Symptoms
Harsh or slipping shifts.
The car goes into limp mode (fixed gear).
Check engine light on.
Common causes
Inadequate transmission fluid level or condition.
Faulty solenoids.
Internal transmission problems.
How to fix it
The first step is to check the level and color of the transmission fluid. Then read the TCM-specific codes with a suitable scanner. Many faults are resolved with a fluid and filter change; others require a specialist workshop.
P0172 indicates that the air-fuel mixture is "rich": too much fuel or too little air in bank 1. The control unit tries to correct it, but if the correction exceeds a limit, it stores the code.
Symptoms
High fuel consumption.
Black smoke from the exhaust.
Rough idle or gasoline smell.
Common causes
Faulty oxygen sensor.
Leaking injector.
Excessive fuel pressure.
Faulty MAF sensor.
How to fix it
Check the oxygen sensors and the MAF sensor, and check the fuel pressure. A prolonged rich mixture damages the catalytic converter and increases fuel consumption.
P0135 refers to the heater circuit of the oxygen sensor (bank 1, sensor 1). That heater speeds up the sensor warm-up so it gives reliable readings on cold starts.
Symptoms
Check engine light on.
Higher fuel consumption.
Possibly elevated emissions.
Common causes
Faulty oxygen sensor.
Blown fuse.
Damaged wiring or connector.
How to fix it
Check the heater fuse first. If it's fine, check the sensor connector and wiring. If everything is correct, replace the oxygen sensor.
The EGR system recirculates part of the exhaust gases into the intake to reduce nitrogen oxide emissions. P0401 appears when the control unit detects that this flow is insufficient.
Symptoms
Check engine light on.
Slight jerking or knocking.
Higher emissions.
Common causes
Faulty or stuck EGR valve.
EGR passages clogged with carbon.
Faulty EGR position sensor.
How to fix it
Remove and clean the EGR valve and its passages; built-up carbon is the most common cause. If the valve doesn't respond, replace it.
P0740 indicates a problem in the torque converter clutch (TCC) circuit, which locks the converter to improve fuel economy on the highway.
Symptoms
The car doesn't "lock" the converter on the highway.
Higher fuel consumption on the freeway.
Check engine light on.
Common causes
Faulty TCC solenoid.
Low or dirty transmission fluid.
Damaged wiring or connector.
How to fix it
Check the level and condition of the transmission fluid. If it's correct, check the TCC solenoid and its connector. In many cases, a fluid and filter change fixes the fault.
P0601 indicates a checksum fault in the internal memory of the engine control module (ECM). It is a serious code because it affects the "brain" of the car.
Symptoms
Check engine light on.
The engine may not start or may run erratically.
Common causes
Faulty ECM.
Software or calibration error.
Low battery voltage.
How to fix it
Check the battery and engine grounds first. If the code persists, it may require reprogramming or replacing the ECM at a specialist workshop.
P0430 indicates that the bank 2 catalytic converter is not filtering the exhaust gases with the expected efficiency. It is the equivalent of P0420 but on the opposite bank of the engine.
Symptoms
Check engine light on.
Slight increase in fuel consumption.
Rotten-egg smell from the exhaust.
Common causes
Worn catalytic converter.
Faulty O2 sensor.
Exhaust leak.
How to fix it
Rule out exhaust leaks and check the O2 sensors before replacing the catalytic converter, which is the most expensive part.
P0455 indicates a significant leak in the EVAP system, which prevents fuel vapors from escaping into the atmosphere. It is usually an easy fault to fix.
Symptoms
Check engine light on.
Gasoline smell.
Common causes
Loose or faulty gas cap.
Cracked EVAP hose.
Faulty purge valve.
How to fix it
Check the gas cap first. If the fault persists, inspect the hoses and valves of the EVAP circuit.