This guide walks you through how to reset the check engine light on a 1999 Mercedes E320 using simple tools and proven methods. Whether you’ve fixed the issue or just want to clear the warning, we’ll help you do it right—without causing damage to your vehicle’s onboard computer.
Key Takeaways
- Understand why the light came on: The check engine light can signal anything from a loose gas cap to a serious engine problem. Diagnosing the root cause is essential before resetting.
- Use an OBD-II scanner for accuracy: A diagnostic tool reads trouble codes and safely resets the light without risking electrical issues.
- Battery disconnection is a last resort: While disconnecting the battery can reset the light, it may also erase radio settings, adaptive transmission data, and other stored information.
- Clear codes only after repairs: Resetting the light without fixing the underlying issue will cause it to reappear—and may lead to further damage.
- Drive cycle matters: After resetting, your car needs to complete a drive cycle so the onboard computer can re-test systems and confirm the problem is resolved.
- Keep records of repairs: Documenting what was fixed helps track recurring issues and supports future maintenance decisions.
- Seek professional help if unsure: If you can’t identify or fix the problem, consult a qualified mechanic to avoid costly mistakes.
Introduction: Why Your 1999 Mercedes E320 Check Engine Light Is On
The check engine light in your 1999 Mercedes E320 isn’t just a warning—it’s your car’s way of saying, “Hey, something needs attention.” Whether it’s a minor issue like a loose gas cap or a more serious problem like a failing oxygen sensor, ignoring it can lead to reduced fuel efficiency, increased emissions, or even engine damage.
But once you’ve diagnosed and fixed the problem, you’ll want to reset that persistent orange glow on your dashboard. Resetting the check engine light isn’t just about aesthetics—it’s about confirming that your repair worked and ensuring your vehicle’s onboard diagnostics (OBD) system is functioning properly.
In this comprehensive guide, you’ll learn exactly how to reset the check engine light on a 1999 Mercedes E320 using safe, reliable methods. We’ll cover everything from understanding OBD-II codes to using a scanner, disconnecting the battery (with caution), and completing a drive cycle. Whether you’re a DIY enthusiast or just want to save a trip to the mechanic, this step-by-step guide will help you get your E320 back to full health.
Understanding the Check Engine Light: What It Means
Visual guide about How to Reset Check Engine Light 1999 Mercedes E320
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Before you reset the light, it’s crucial to understand why it came on in the first place. The 1999 Mercedes E320 is equipped with an OBD-II (On-Board Diagnostics II) system, which monitors engine performance, emissions, and various sensors. When something goes outside normal parameters, the system stores a trouble code and triggers the check engine light.
Common reasons for the light include:
- A loose or faulty gas cap
- Faulty oxygen (O2) sensor
- Misfiring spark plugs or ignition coils
- Catalytic converter issues
- Mass airflow (MAF) sensor problems
- Evaporative emissions (EVAP) system leaks
Each of these issues triggers a specific diagnostic trouble code (DTC), such as P0440 (EVAP system leak) or P0300 (random misfire). These codes help pinpoint the problem so you can fix it correctly.
Important: Resetting the light without addressing the root cause is like turning off a smoke alarm while the fire still burns. The light will likely return, and the underlying issue could worsen over time.
Tools You’ll Need to Reset the Check Engine Light
To reset the check engine light on your 1999 Mercedes E320, you’ll need a few basic tools. The method you choose will determine what you need:
Option 1: OBD-II Scanner (Recommended)
- OBD-II diagnostic scanner (available at auto parts stores or online for $20–$100)
- Access to the OBD-II port (located under the dashboard, usually near the driver’s side knee panel)
Option 2: Battery Disconnection (Alternative Method)
- Wrench or socket set (typically 10mm or 13mm for battery terminals)
- Safety gloves and glasses
- Optional: Memory saver device (to preserve radio and ECU settings)
Optional but Helpful Tools
- Flashlight (to see under the dashboard)
- Owner’s manual (for fuse box and battery location)
- Notebook or phone to record trouble codes
Investing in a basic OBD-II scanner is highly recommended. It not only helps you read codes but also safely clears them—without risking data loss or electrical issues.
Step 1: Diagnose the Problem Using an OBD-II Scanner
The first and most important step is to find out why the check engine light is on. Skipping this step and resetting the light blindly can lead to repeated warnings and potential damage.
Locate the OBD-II Port
In the 1999 Mercedes E320, the OBD-II port is typically located under the dashboard on the driver’s side, near the steering column. You may need to crouch down and look up to spot it. It’s a 16-pin connector, usually black or gray, and shaped like a trapezoid.
Connect the Scanner
Turn off the ignition, then plug the OBD-II scanner into the port. Make sure it’s fully seated. Some scanners power on automatically when connected; others require you to turn the ignition to the “ON” position (without starting the engine).
Read the Trouble Codes
Follow the scanner’s instructions to retrieve the diagnostic trouble codes (DTCs). Most scanners will display codes like “P0420” or “P0171.” Write these down or take a photo for reference.
Interpret the Codes
Use the scanner’s built-in code library or search online for what each code means. For example:
- P0420: Catalyst system efficiency below threshold (likely catalytic converter issue)
- P0171: System too lean (possibly a vacuum leak or MAF sensor problem)
- P0455: Large EVAP leak (often a loose gas cap)
Once you know the cause, you can decide whether to fix it yourself or take it to a mechanic. Only proceed to reset the light after the issue is resolved.
Step 2: Fix the Underlying Issue
Now that you’ve identified the problem, it’s time to fix it. Here are some common fixes for the 1999 E320:
Tighten or Replace the Gas Cap
If the code points to an EVAP leak (like P0455), start with the gas cap. Turn it clockwise until it clicks 2–3 times. If it’s cracked, worn, or missing the seal, replace it with an OEM or high-quality aftermarket cap.
Replace Faulty Sensors
Oxygen sensors and MAF sensors are common culprits. These parts degrade over time and can cause poor fuel economy and emissions. Replacement is straightforward with basic tools:
- O2 sensors are usually located on the exhaust manifold or downpipe.
- MAF sensors are near the air filter box and can often be cleaned with MAF cleaner spray.
Address Misfires
A P0300 code indicates random misfires. Check spark plugs, ignition coils, and wires. The 1999 E320 uses coil-on-plug ignition, so each cylinder has its own coil. Replace any cracked or worn components.
Check for Vacuum Leaks
Hissing sounds under the hood or a lean fuel code (P0171/P0174) may indicate a vacuum leak. Inspect hoses for cracks, especially around the intake manifold and PCV system.
After making repairs, double-check all connections and ensure everything is properly tightened.
Step 3: Reset the Check Engine Light Using an OBD-II Scanner
Once the issue is fixed, it’s time to reset the check engine light. The safest and most effective way is using your OBD-II scanner.
Power On the Scanner
With the scanner still connected, turn the ignition to the “ON” position (do not start the engine). The scanner should power up and display menu options.
Navigate to the Clear Codes Option
Use the scanner’s buttons to find the “Clear Codes,” “Erase Codes,” or “Reset DTCs” option. This may be under a menu like “Diagnostics” or “Service.”
Confirm the Reset
The scanner will ask you to confirm. Select “Yes” or “OK.” The device will send a command to the car’s ECU (Engine Control Unit) to clear all stored trouble codes and turn off the check engine light.
Verify the Light Is Off
Turn the ignition off, then back on. The check engine light should no longer be illuminated. If it comes back on immediately, the problem may not be fully resolved, or a new issue has occurred.
Pro Tip: Some scanners also allow you to view live data (like engine RPM, coolant temperature, and oxygen sensor readings). Use this feature to confirm your repair worked before clearing codes.
Step 4: Alternative Method – Disconnecting the Battery
If you don’t have an OBD-II scanner, you can reset the check engine light by disconnecting the battery. However, this method has drawbacks and should be used cautiously.
Why Battery Disconnection Works
The car’s ECU stores trouble codes in volatile memory, which requires power to maintain. Disconnecting the battery cuts power, forcing the ECU to reset and clear stored codes—including the check engine light.
Steps to Disconnect the Battery
- Turn off the ignition and remove the key.
- Open the hood and locate the battery (in the 1999 E320, it’s in the engine bay on the driver’s side).
- Put on safety gloves and glasses.
- Use a wrench to loosen the negative (-) terminal clamp first (black cable). Remove it and tuck it away from the battery post.
- Next, disconnect the positive (+) terminal (red cable).
- Wait at least 15–30 minutes. This allows the ECU to fully discharge and reset.
- Reconnect the positive terminal first, then the negative. Tighten securely.
- Close the hood and start the car.
What to Expect After Reconnection
The check engine light should be off. However, you may notice:
- The radio asks for a security code (check your owner’s manual or glove box for the code).
- The climate control or seat memory settings are reset.
- The transmission may feel “rough” for a few drives as the adaptive learning resets.
To avoid these issues, consider using a memory saver device that plugs into the OBD-II port or cigarette lighter to maintain power during the reset.
Step 5: Complete a Drive Cycle
Resetting the light is only half the battle. Your car’s onboard computer needs to re-test all systems to ensure the problem is truly fixed. This process is called a “drive cycle.”
A drive cycle is a specific sequence of driving conditions that allows the ECU to monitor emissions and engine performance. Without completing it, the check engine light may not stay off, or the car may fail an emissions test.
Typical Drive Cycle for 1999 Mercedes E320
Follow these steps over the course of 1–2 days:
- Start the engine when cold (after sitting overnight). Let it idle for 2–3 minutes.
- Drive at 30–40 mph for 5 minutes with steady acceleration.
- Accelerate to 55–60 mph and maintain speed for 10 minutes.
- Decelerate to 20 mph without braking hard (coast down).
- Stop and idle for 2 minutes.
- Repeat the cycle 2–3 times.
During this time, the ECU will run tests on the oxygen sensors, catalytic converter, EVAP system, and more. If all tests pass, the readiness monitors will show “complete,” and the check engine light should remain off.
Check Readiness Monitors
You can use your OBD-II scanner to check “readiness monitors” after the drive cycle. These indicate which systems have been tested. For emissions testing, most states require at least 2–3 monitors to be complete.
Troubleshooting Common Issues
Even after following all steps, you might run into problems. Here’s how to handle them:
The Check Engine Light Comes Back On
If the light returns shortly after resetting, the original problem wasn’t fully fixed. Re-scan for codes and double-check your repairs. Sometimes, a part may appear functional but still fail under load.
Radio or Settings Are Reset
This happens after battery disconnection. Re-enter the radio code (usually found in the owner’s manual or glove box). Reset seat and mirror positions if needed.
Car Runs Rough After Reset
The transmission and engine may need time to relearn driving patterns. Drive normally for 50–100 miles. If issues persist, have the ECU checked for faults.
Scanner Won’t Connect
Ensure the ignition is on, the scanner is compatible with OBD-II, and the port isn’t damaged. Try cleaning the port with electrical contact cleaner.
No Codes Are Found, But Light Is On
Some intermittent issues may not store a code immediately. Drive the car for a few days and re-scan. If the light stays on, consider a professional diagnostic.
When to See a Mechanic
While many check engine light issues can be handled at home, some problems require expert attention. Seek a qualified Mercedes technician if:
- You’re unsure how to interpret or fix the trouble code.
- The light flashes (indicating a severe misfire that can damage the catalytic converter).
- You’ve tried multiple fixes, but the light keeps returning.
- The car is running poorly, overheating, or making unusual noises.
Mercedes-Benz vehicles have complex electronics, and improper repairs can lead to costly damage. When in doubt, it’s better to pay for professional diagnostics than risk making the problem worse.
Maintaining Your 1999 Mercedes E320
Preventing future check engine light issues starts with regular maintenance:
- Replace spark plugs and coils every 60,000–100,000 miles.
- Use high-quality fuel and change the fuel filter as recommended.
- Inspect and replace the air filter regularly.
- Check the gas cap for tightness and wear.
- Use a fuel system cleaner every 10,000 miles to prevent carbon buildup.
- Keep up with scheduled services, including oil changes and transmission fluid checks.
A well-maintained E320 can run smoothly for over 200,000 miles. Regular care reduces the chances of unexpected check engine light surprises.
Conclusion: Take Control of Your Car’s Health
Resetting the check engine light on your 1999 Mercedes E320 doesn’t have to be intimidating. With the right tools and knowledge, you can diagnose, repair, and clear the warning safely and effectively. Remember: the light is there to protect your car, so always address the root cause before resetting.
Using an OBD-II scanner is the best method—it’s accurate, safe, and gives you valuable diagnostic information. Battery disconnection works in a pinch but comes with trade-offs. And don’t forget the drive cycle—it’s essential for confirming your repair and keeping the light off for good.
By following this guide, you’re not just turning off a warning light—you’re taking control of your vehicle’s health, saving money on diagnostics, and gaining confidence in your DIY skills. Your 1999 E320 has decades of life left; treat it well, and it will reward you with reliable, luxurious driving.