What to Do When the Car's Check Engine Light Goes On

Photo is of a piece of art that turns heads anywhere you go but expensive to maintain. I meant the Nissan Fairlady.

The check engine light is one of the most concerning warnings that can appear on your vehicle’s dashboard. It can indicate anything from a minor issue, such as a loose gas cap, to a serious engine problem that requires immediate attention. Knowing what to do when this light comes on can help prevent further damage and costly repairs.

1. Don’t Panic – Assess the Situation

When the check engine light turns on, the first thing to do is stay calm. The light does not necessarily mean your car is about to break down immediately. However, it is a sign that something needs attention.

Look for other warning signs, such as unusual noises, reduced performance, strange smells, or excessive smoke from the exhaust. If the car is running normally, you likely have time to diagnose the issue before taking further action.

2. Check If the Light Is Flashing or Steady

• A Steady Light: This means there is a problem that needs to be checked, but it is not an emergency. You can continue driving cautiously and schedule a diagnostic check soon.
• A Flashing Light: A flashing check engine light usually indicates a serious issue, such as a misfiring engine, which can lead to major damage. If this happens, reduce speed and avoid hard acceleration. In severe cases, pull over and call for assistance.

3. Check the Gas Cap

One of the most common reasons for a check engine light is a loose or faulty gas cap. If the cap is not properly secured, it can cause fuel vapors to escape, triggering the light. Simply remove the gas cap, check for damage, and screw it back on tightly. If the light does not turn off after driving for a while, the problem may be something else.

4. Use an OBD-II Scanner to Diagnose the Issue

A quick way to find out why the check engine light is on is by using an OBD-II scanner. Most auto parts stores offer free diagnostic scans, or you can purchase your own scanner. The device plugs into your car’s onboard diagnostics port and retrieves an error code that corresponds to a specific problem.

5. Look Up the Error Code

Once you have the error code from the scanner, search online or consult your vehicle’s manual to understand what it means. The error codes are standard for any car.  Some common check engine light codes include:

• P0171/P0174: Engine running too lean
• P0300: Engine misfire
• P0420: Catalytic converter issue
• P0442: Small EVAP system leak (possibly a gas cap issue)

6. Determine If You Need Immediate Repairs

Some issues, like a loose gas cap, can be easily fixed without professional help. Others, like a bad oxygen sensor, failing spark plugs, or a catalytic converter problem, require a mechanic. If your car shows signs of trouble—like rough idling, poor acceleration, or excessive smoke—it’s best to stop driving and get it checked immediately.

7. Reset the Check Engine Light (If the Issue Is Fixed)

If you have addressed the problem, you can reset the check engine light using an OBD-II scanner. If the light turns back on, the issue may not be fully resolved, and you should consult a professional mechanic.

Final Thoughts

The check engine light should never be ignored, but it also doesn’t always mean a disaster. By staying calm, checking for simple fixes, and using diagnostic tools, you can determine whether the issue is minor or serious. When in doubt, consult a trusted mechanic to ensure your car remains in good condition.

Here are 20 common OBD-II error codes along with their meanings:

Engine and Fuel System Issues

1. P0171 – System Too Lean (Bank 1)
2. P0172 – System Too Rich (Bank 1)
3. P0174 – System Too Lean (Bank 2)
4. P0175 – System Too Rich (Bank 2)

Misfire and Ignition Issues

5. P0300 – Random/Multiple Cylinder Misfire Detected
6. P0301 – Cylinder 1 Misfire Detected
7. P0302 – Cylinder 2 Misfire Detected
8. P0303 – Cylinder 3 Misfire Detected
9. P0304 – Cylinder 4 Misfire Detected

Oxygen Sensor and Emissions Issues

10. P0131 – O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
11. P0135 – O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
12. P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
13. P0430 – Catalyst System Efficiency Below Threshold (Bank 2)

Fuel and Air Intake System Issues

14. P0442 – Small Leak Detected in EVAP System
15. P0455 – Large Leak Detected in EVAP System (e.g., loose gas cap)
16. P0507 – Idle Air Control System RPM Higher Than Expected

Transmission and Powertrain Issues

17. P0700 – Transmission Control System Malfunction
18. P0730 – Incorrect Gear Ratio

Throttle and Sensor Issues

19. P0128 – Coolant Thermostat Malfunction (Coolant Temperature Below Regulating Temperature)
20. P0220 – Throttle/Pedal Position Sensor B Circuit Malfunction