Engine Light On: What Now? Causes and Solutions [2025]

The check engine light (also known as the check engine light or engine management light) is one of the most frightening warnings on your dashboard. When this orange or yellow light comes on, almost every car owner gets an uneasy feeling. Does this mean the engine is about to fail? Should you stop immediately? Or can you keep driving? The truth is, the check engine light can be triggered by anything from a loose gas cap to serious engine problems. In this complete guide for 2025, you'll discover the most common causes, when you should be concerned, and what the next steps are.

What Does the Engine Light Actually Mean?

The check engine light is connected to your car's engine management system (ECU). This computer constantly monitors hundreds of sensors and systems.

How does the engine light system work?

• Continuous monitoring: The ECU controls emissions, fuel mixture, ignition, air supply, exhaust system and dozens of other parameters
• Error code registration: When a value falls outside normal limits, the ECU stores a DTC (Diagnostic Trouble Code)
• Light activates: The engine light will come on to warn you
• Error code remains stored: Even if the light goes out later, the error code remains in memory

Different lamp behaviors:

• Constantly lit (orange/yellow): A problem has been detected, but usually not critical. You can often still drive, but you will need to have a diagnosis performed.
• Flashing: Serious problem, often ignition problems. Reduce speed and visit a garage immediately.
• Red: Critical problem. Stop safely and turn off the engine. Call roadside assistance.
• Turns on and off again: Intermittent problem. Still needs to be investigated.

Bee Faay Auto Group We can read and interpret the exact error codes using professional diagnostic equipment.

12 Most Common Causes of Engine Light

1. Loose or defective fuel cap (15-20% for all cases)

The most common and harmless cause. The fuel system is under pressure and must be closed. A loose fuel cap causes a loss of pressure.

• Symptoms: Engine light only, no performance issues
• Error code: P0440, P0455, P0456 (EVAP system leak)
• Solution: Check that the fuel cap is properly tightened (it should click). Replace if damaged (€15-€30).
• Costs: €0-€30

2. Faulty lambda probe / oxygen sensor (10-15% from cases)

Lambda sensors measure the oxygen in the exhaust gases to optimize the fuel mixture. Most cars have two to four lambda sensors.

• Symptoms: Increased fuel consumption, reduced performance, rough idle
• Error code: P0130-P0167 (various lambda probe codes)
• Solution: Replace lambda probe
• Costs: €150-€400 per sensor including labor

Ignoring it can lead to catalytic converter damage (€500-€1,500).

3. Faulty catalytic converter (8-12% of fallen)

The catalytic converter cleans exhaust gases. It can fail due to age, incorrect fuel, or ignoring other problems.

• Symptoms: Loss of power, rattling noise from exhaust, poor acceleration, increased emissions
• Error code: P0420, P0430 (catalyst efficiency)
• Solution: Replace catalytic converter (sometimes repairable)
• Costs: €500-€1.500

4. Mass airflow sensor (MAF) faulty (5-8% from dropped)

The MAF sensor measures how much air the engine takes in, crucial for the correct fuel-to-air ratio.

• Symptoms: Poor idling, engine stuttering, reduced performance, difficult starting
• Error code: P0100-P0104 (MAF sensor)
• Solution: First clean with MAF cleaner (€10), otherwise replace
• Costs: €10 (cleaning) to €200-€400 (replacement)

5. Spark plugs or coils defective (5-8% of cases)

Worn spark plugs or defective ignition coils cause ignition problems.

• Symptoms: Engine runs irregularly, vibration, loss of power, engine light flashes frequently
• Error code: P0300-P0308 (misfire – ignition error)
• Solution: Replace spark plugs and/or coils
• Costs: €120-€350 depending on number of cylinders

At a major maintenance service Spark plugs are replaced as standard.

6. Clogged air filter (3-5% from fallen)

A clogged air filter restricts airflow to the engine.

• Symptoms: Reduced performance, increased fuel consumption, black smoke
• Error code: Often P0171, P0174 (too lean mixture)
• Solution: Replace air filter
• Costs: €30-€80

7. Vacuum leakage (3-5% of cases)

Leaks in vacuum hoses or intake manifold will disrupt the air-fuel ratio.

• Symptoms: High idle speed, whistling noise, poor idle
• Error code: P0171, P0174 (system too lean)
• Solution: Locate and repair leak
• Costs: €50-€300 depending on location

8. EGR valve failure (3-5% of cases)

The EGR (Exhaust Gas Recirculation) valve recycles exhaust gases to reduce emissions. It can become clogged due to soot.

• Symptoms: Power loss, rough idle, black smoke (diesel)
• Error code: P0400-P0409 (EGR problems)
• Solution: Cleaning or replacing the EGR valve
• Costs: €150-€500

9. Fuel pump or pressure sensor problem (2-4% of cases)

• Symptoms: Starting problems, engine hesitation when accelerating, loss of power
• Error code: P0087, P0088, P0230-P0233
• Solution: Replace fuel pump or pressure sensor
• Costs: €300-€800

10. Turbo problems (2-3% of cases, mainly diesels)

• Symptoms: Lack of power, whistling sound, blue smoke, oil consumption
• Error code: P0234-P0238 (turbo/boost pressure)
• Solution: Repair or replace turbo
• Costs: €800-€2.500

11. Clogged particulate filter (diesel car, 5-10% dropped from diesel)

• Symptoms: Loss of power, emergency mode, increased fuel consumption
• Error code: P242F, P2002, P2003
• Solution: Regenerate or replace particulate filter
• Costs: €100-€200 (regeneration) or €800-€2,000 (replacement)

12. Thermostat failure (2-3% of fallen)

• Symptoms: Engine does not reach operating temperature, increased consumption, poor heating
• Error code: P0128 (coolant temperature too low)
• Solution: Replace thermostat
• Costs: €150-€350

When Should You Stop Immediately?

In these cases you must immediately park the car safely and switch off the engine:

• Engine light flashing: Indicates serious ignition problems that can damage the catalytic converter.
• Red warning lights: Oil pressure, temperature, battery – these are critical issues
• Strange noises: Knocking, banging or metal-on-metal noise from the engine
• Smoke or burning smell: May indicate overheating or fire
• Severe loss of power: Engine goes into emergency mode
• Vibration or shock: Engine runs very irregularly

In these cases: switch off the engine, call roadside assistance (ANWB, Europeesche, etc.) and do not drive the car any further.

What to Do When the Engine Light Is On?

Step 1: Observe and feel

• Is the light on constantly or is it flashing?
• Are there any other warning lights?
• Does the engine feel different (vibration, loss of power)?
• Do you hear strange noises?

Step 2: Check the obvious

• Is the fuel cap tightened properly?
• Has the tank been filled recently? (Wrong fuel?)
• Have there been any recent repairs?

Step 3: Drive safely to the garage (if the light is on constantly)

• Avoid high revs and speeds
• Pay attention to changes in behavior
• Schedule a diagnostic appointment quickly (within 1-2 days)

Step 4: Professional diagnosis

Visit a garage for an error code reading. At Faay Auto Groep, we offer:

• Complete error code analysis with professional equipment
• Interpretation of codes (not all codes are what they seem)

• Live data analysis to find intermittent problems

• Transparent price quote for repairs
• Warranty on repairs carried out

Cost of diagnosis:

• Basic diagnosis (read code): €30-€60
• Extensive diagnosis with tests: €60-€120
• DIY code reader (OBD2 scanner): €20-€80 (basic codes only, no interpretation)

Please note: Free code readers at some auto parts stores only provide the code, not a professional interpretation.

Frequently Asked Questions

Can I continue driving with the engine light on?

If the check engine light is on constantly (not flashing) and the car drives normally, you can often still drive. However, schedule a diagnostic appointment within 1-2 days. Prolonged driving with the check engine light on can lead to further damage to components, increased fuel consumption, possible MOT rejection, and potentially (very expensive) catalytic converter damage. If the light is flashing, stop immediately. If in doubt, err on the side of caution and have it checked immediately.

Can I reset the engine light myself?

Technically yes, you can clear codes with an OBD2 scanner. However, this is strongly discouraged without first repairing the problem. Reasons: the light will return if the problem isn't fixed, you may be masking serious problems that will get worse, clearing codes also removes valuable diagnostic information a mechanic needs, and some cars require a "drive cycle" before all systems are retested (which can take days). The only valid reason to reset: after a repair to verify that the problem has been resolved.

Why does the engine light sometimes go out by itself?

This happens with intermittent problems—problems that come and go. Examples include a sensor that's making poor contact due to vibration, a problem that only occurs under certain conditions (cold, warm, high RPM), or a near-miss error that occasionally falls just outside normal limits. Important: even if the light goes out, the error code remains stored in memory as a "pending code" or "history code." This helps a mechanic diagnose the problem. So don't ignore a light that goes out—have it diagnosed anyway, as the problem still exists.

What is the difference between OBD1 and OBD2?

OBD (On-Board Diagnostics) is the diagnostic system in cars. OBD1 was used until the mid-1990s and had brand-specific codes and connectors – each manufacturer had its own system. OBD2 became mandatory in 1996 (USA) and 2001 (Europe) and has standardized codes (P-codes) and a universal 16-pin connector. Almost all cars manufactured after 2001 have OBD2, which means that any generic OBD2 scanner can read basic codes. For more extensive diagnostics, brand-specific scanners are often still required. Check the year of manufacture of your car – after 2001, you almost certainly have OBD2.

Can a dead battery trigger the engine light?

Not directly, but indirectly, effects are possible. Scenario: Battery nearly discharges, voltage drops below a critical level (below 11V), ECU and sensors receive unstable voltage, which can trigger error codes (often false codes). After charging or replacing the battery, codes remain in memory. Also, disconnecting or replacing the battery can cause the engine light to come on because adaptive values (learned settings) have been erased – the ECU needs to relearn. Solution: After battery problems or replacement, have the codes cleared and drive for a few days. If the light returns, there is a real problem.

Will my car pass its MOT with the engine light on?

No, absolutely not. An illuminated engine light is an automatic rejection at the MOT inspection. The inspector checks: whether the engine light comes on when the ignition is on (system test), whether the light goes out after starting (normal behavior), and whether the light comes on while idling (rejection). OBD readings are also performed on modern cars – stored error codes can lead to rejection, even if the light is off. Schedule repairs at least 1-2 weeks before the MOT date – some systems need to run a drive cycle before they are "ready" for the MOT.

Conclusion: Never Ignore the Engine Light

An illuminated engine light is your car's way of signaling that attention is needed. While the cause can range from something as minor as a loose gas cap to serious engine problems, it's always wise to have it checked out promptly. Early diagnosis prevents minor issues from escalating into expensive repairs. A €200 lambda sensor ignored can damage a €1,500 catalytic converter.

Key advice: observe whether the light is on constantly or flashing, check the obvious first (fuel cap), drive carefully to a garage if the light is on constantly, and stop immediately and call roadside assistance if the light flashes or red lights are also on.

Engine light on? Schedule a professional diagnosis appointment with Faay Auto Groep today. We'll read the error codes, diagnose the problem, and give you a fair quote. We can often fix the problem the same day. Call 030 601 99 15 or book your diagnosis appointment online. We're ready to assist you in Nieuwegein!