P0420, P0171, P0300 and Other Popular OBD2 Error Codes: What They Mean and What to Do

When the check engine light comes on, your heart beats faster. How much will the repair cost? Can you keep driving? What even broke?

The first thing to do is read the error code through an OBD2 adapter. But numbers like P0420 or P0171 don't say much on their own. In this article, we'll break down the most common codes car owners encounter, explain in plain language what they mean, and show how AIscanAuto helps you understand the problem in minutes instead of hours of internet searching.

How Error Codes (DTC) Are Structured

Every code consists of one letter and four digits:

• P — Powertrain (engine and transmission). The most common.
• B — Body (bodywork, interior, electrics).
• C — Chassis (suspension, brakes).
• U — Network (communication between control modules).

The digits specify the system and the exact fault. Below we'll cover the top P-category codes that most drivers face.

P0420 — Catalytic Converter Efficiency Below Threshold

What it means: the engine control module (ECM) sees that the catalytic converter isn't cleaning exhaust gases well enough. It usually compares the readings from two oxygen sensors — before and after the converter.

Symptoms:
- check engine light is on;
- possibly slight loss of power;
- smell of hydrogen sulfide ("rotten eggs") from the exhaust.

Causes:
- failed catalytic converter (burned out, broken apart);
- faulty oxygen sensor;
- air leak in the intake;
- ignition or fuel system problems that "kill" the catalytic converter.

What to do:
1. Check the oxygen sensors — they're cheaper than the catalytic converter.
2. Check spark plugs and ignition coils. Misfires flood the catalytic converter with fuel.
3. Inspect the exhaust for leaks.
4. If the catalytic converter is burned out — repair or replace it. Removing the catalytic converter and installing a spacer (defouler) is illegal in most countries and harms the environment.

AIscanAuto shows live data from oxygen sensors in real time — you can immediately see whether the problem is the sensor or the catalytic converter.

P0171 — System Too Lean (Bank 1)

What it means: the engine is getting too much air or too little fuel. The mixture is "lean" — there's not enough fuel for proper combustion.

Symptoms:
- unstable idle;
- hesitation when pressing the accelerator;
- possible increased fuel consumption (paradoxically, the ECM tries to compensate).

Causes:
- air leak (crack in a hose, leaking intake manifold gasket);
- dirty air filter;
- faulty mass air flow (MAF) sensor;
- weak fuel pressure (fuel pump, filter, regulator);
- clogged fuel injectors.

What to do:
1. Visually inspect all intake hoses — cracks and loose clamps are visible to the naked eye.
2. Clean or replace the MAF sensor.
3. Check fuel rail pressure.
4. Reset adaptations after repair.

AIscanAuto displays fuel trim in real time — if the values are far from zero, the mixture is definitely lean or rich.

P0300 — Random/Multiple Cylinder Misfire Detected

What it means: the engine is "misfiring" — fuel isn't igniting in one or more cylinders. P0300 means the cylinders vary (random misfires), while P0301–P0308 point to a specific cylinder.

Symptoms:
- shaking at idle;
- loss of power;
- knocking from the exhaust pipe;
- smell of gasoline from the exhaust.

Causes:
- worn or fouled spark plugs;
- faulty ignition coils or high-voltage wires;
- bad fuel;
- low compression in cylinders;
- fuel injector problems.

What to do:
1. Check and replace spark plugs — the cheapest and most common culprit.
2. Swap ignition coils and see if the error moves to another cylinder.
3. Check compression if spark plugs and coils are fine.
4. Check fuel injectors for leaks and spray pattern.

AIscanAuto shows live misfire count data per cylinder — you can pinpoint the problem before visiting the repair shop.

Other Popular Codes

P0135 — O2 Sensor Heater Circuit (Bank 1, Sensor 1)

What it means: the oxygen sensor isn't heating up to operating temperature. On a cold engine, the ECM works "blind," which worsens fuel consumption and emissions.

What to do: replace the oxygen sensor. Often the cause is broken wiring or a burnt heater inside the sensor.

P0442 — Small Leak Detected in EVAP System

What it means: the evaporative emission control system detected a small leak. Most often the culprit is a gas cap that isn't tightened all the way.

What to do: check the fuel cap, then inspect EVAP hoses for cracks. If that doesn't help — check the purge valve.

P0401 — Insufficient EGR Flow

What it means: the exhaust gas recirculation valve isn't letting enough gases through. Leads to higher combustion temperatures and increased NOx emissions.

What to do: clean the EGR valve and passages. Sometimes simply removing carbon buildup is enough.

When You Can Drive and When You Should Stop

You can drive carefully: P0420 (if there's no power loss), P0442, P0135 (but the engine runs in safe mode).

Better to stop: P0300 (engine is misfiring — you can damage the catalytic converter), P0171 with strong vibration, any code with a flashing check engine light (flashing "check" = active misfires that threaten catalytic converter damage).

How AIscanAuto Helps with Error Codes

Instead of guessing from the code, AIscanAuto does three things:

1. Reads codes — shows not only the number but also the text description in your language.
2. AI explains — the neural network translates technical language into plain words: "this means that...", "most likely the cause is...", "check first..."
3. Shows live data — fuel trim, oxygen sensor readings, misfire counts, temperatures, RPM — all in one window in real time.

This way you arrive at the repair shop not with "my check engine is on," but with specific data: "P0171, fuel trim +25%, MAF reading below normal." The mechanic immediately understands the problem, and you save time and money.

First Steps Checklist When an Error Appears

1. Connect AIscanAuto and read the codes.
2. Write down all active codes and their descriptions.
3. Check live data — fuel trim, oxygen sensors, coolant temperature.
4. Look under the hood — visually inspect hoses, spark plugs, connectors.
5. Clear the error after fixing the cause and make sure it doesn't come back.

Conclusion

Error codes aren't a death sentence — they're a diagnosis. P0420 doesn't always mean a dead catalytic converter, P0171 is often fixed by replacing a hose, and P0300 — by changing spark plugs. The key is to understand what the car is saying and act methodically.

With AIscanAuto, diagnostics become understandable even without deep technical knowledge. The program is free, works on any platform, and turns abstract numbers into concrete actions.

FAQ

Q: Can I just clear the error and forget about it?
A: If the cause isn't fixed — the error will return after a few trips. Clearing without repair is only a temporary measure.

Q: How many codes can the ECM store at once?
A: Depends on the car, usually several dozen. AIscanAuto will show all active and stored codes.

Q: Will AIscanAuto tell me the exact cause of the error?
A: The program gives the most likely causes based on the code and live data. An exact diagnosis requires physical inspection, but AIscanAuto narrows the search significantly.