Resetting the check engine light on a Volvo 940 can be done manually or with a scan tool after fixing the underlying issue. This guide walks you through both methods, safety tips, and common pitfalls to avoid.
Key Takeaways
- Diagnose first: Never reset the light without identifying and fixing the root cause—use an OBD-I scanner or manual code reading.
- Volvo 940 uses OBD-I: This older system requires specific tools or manual methods, unlike modern OBD-II vehicles.
- Battery disconnect method works: Disconnecting the battery for 10–15 minutes can reset the ECU and clear codes—but may also reset radio and clock settings.
- Manual code clearing is possible: Some models allow clearing codes by grounding specific pins in the diagnostic connector.
- Use a compatible scanner: Invest in an OBD-I scanner that supports Volvo 940 (1991–1995) for accurate diagnostics and resets.
- Recheck after reset: Drive the car for a few days—if the light returns, the problem persists and needs further attention.
- Prevent future issues: Regular maintenance (spark plugs, oxygen sensor, fuel system) reduces check engine light triggers.
How to Reset Check Engine Light Volvo 940: A Complete Guide
If you own a classic Volvo 940—whether it’s a 1991, 1992, 1993, 1994, or 1995 model—you know these cars are built to last. But like any older vehicle, they can develop issues that trigger the dreaded check engine light. When that little yellow or orange light pops up on your dashboard, it’s your car’s way of saying, “Hey, something’s not right under the hood.”
The good news? You don’t always need a mechanic to reset the check engine light on your Volvo 940. With the right tools and a bit of know-how, you can do it yourself—safely and effectively. This guide will walk you through everything you need to know: from diagnosing the problem to clearing the code and preventing future warnings.
Whether you’re a DIY enthusiast or just trying to save a few bucks on labor costs, this step-by-step tutorial will help you reset the check engine light on your Volvo 940 with confidence. We’ll cover manual methods, scanner tools, troubleshooting tips, and best practices to keep your classic Volvo running smoothly.
Let’s get started.
Understanding the Volvo 940 Check Engine Light
Visual guide about How to Reset Check Engine Light Volvo 940
Image source: youcanic.com
Before you reset the check engine light, it’s important to understand what it means—and why it’s on in the first place.
The Volvo 940, produced from 1991 to 1995, uses an On-Board Diagnostics I (OBD-I) system. Unlike modern cars with OBD-II (standardized since 1996), OBD-I systems are manufacturer-specific and less universal. That means the way you read and clear codes is different—and sometimes a bit trickier.
When the check engine light illuminates, it means the Engine Control Unit (ECU) has detected a fault in the engine management system. This could be anything from a loose gas cap to a failing oxygen sensor, a misfiring spark plug, or a problem with the fuel injection system.
Important: Resetting the light without fixing the underlying issue is like turning off a smoke alarm while your house is still on fire. The light will likely come back on—and the problem could get worse.
So, the first step isn’t to reset the light—it’s to figure out why it’s on.
Step 1: Diagnose the Problem
You can’t fix what you don’t understand. Before resetting the check engine light on your Volvo 940, you need to read the trouble codes stored in the ECU.
There are two main ways to do this: manually or with a scanner.
Method A: Manual Code Reading (No Tools Required)
If you don’t have a scanner, you can still read the codes using the car’s built-in diagnostic system. Here’s how:
Step 1: Locate the diagnostic connector. On the Volvo 940, it’s usually found under the dashboard on the driver’s side, near the fuse box. It’s a small, rectangular connector with multiple pins.
Step 2: Turn the ignition to the “ON” position (but don’t start the engine).
Step 3: Use a paperclip or small jumper wire to connect pin A to pin B in the diagnostic connector. (Refer to your owner’s manual or a Volvo 940 wiring diagram to confirm pin locations—these are typically labeled.)
Step 4: Watch the check engine light. It will begin to flash in a pattern that corresponds to the trouble code.
For example:
– One long flash, then three short flashes = Code 13
– Two long flashes, then four short flashes = Code 24
Each code corresponds to a specific issue. Common Volvo 940 codes include:
– Code 13: Oxygen sensor circuit malfunction
– Code 24: Engine coolant temperature sensor issue
– Code 32: Mass airflow (MAF) sensor problem
– Code 44: Evaporative emissions system leak
Write down the code(s) and look them up in a Volvo 940 service manual or online database.
Method B: Use an OBD-I Scanner
For a faster, more reliable diagnosis, use an OBD-I scanner that’s compatible with Volvo 940 models. These scanners plug into the diagnostic port and display the code(s) on a screen.
Look for scanners that specifically mention Volvo or European OBD-I support. Some popular options include:
– OBD-I to USB adapters with Volvo-compatible software
– Handheld scanners like the Actron CP9575 (with Volvo adapter)
– Bluetooth OBD-I readers paired with apps like Torque (Pro version)
Once connected, the scanner will read the codes and often provide a brief description of the issue. This makes it easier to pinpoint the problem before attempting a reset.
Step 2: Fix the Underlying Issue
Now that you know why the check engine light is on, it’s time to fix it.
Common causes in the Volvo 940 include:
– Faulty oxygen sensor (very common in older Volvos)
– Loose or damaged gas cap
– Worn spark plugs or ignition wires
– Dirty or failing mass airflow (MAF) sensor
– Vacuum leaks in the intake system
– Engine coolant temperature sensor failure
For example, if you’ve got Code 13 (oxygen sensor), you’ll need to inspect the sensor located in the exhaust manifold. Over time, these sensors degrade and send incorrect signals to the ECU, triggering the light.
Replacing an oxygen sensor on a Volvo 940 is a moderate DIY job. You’ll need:
– A new oxygen sensor (make sure it’s compatible with your model year)
– A oxygen sensor socket or wrench
– Penetrating oil (like WD-40) if the sensor is stuck
– Safety gloves and goggles
Similarly, if the code points to a loose gas cap, simply tighten it or replace it with a new OEM-style cap. This is one of the easiest fixes—and one of the most overlooked.
Always address the root cause before resetting the light. Otherwise, you’re just masking the problem.
Step 3: Reset the Check Engine Light
Once the issue is fixed, you can safely reset the check engine light. There are three main methods: battery disconnect, manual code clearing, and using a scanner.
Method 1: Disconnect the Battery (Simplest Method)
This is the most common and accessible way to reset the ECU and clear the check engine light on a Volvo 940.
Step 1: Turn off the engine and remove the key.
Step 2: Open the hood and locate the battery. The Volvo 940 typically has a 12-volt lead-acid battery in the engine bay.
Step 3: Use a wrench to loosen the negative (black) battery terminal. Remove it completely from the battery post.
Step 4: Wait 10 to 15 minutes. This allows the ECU to fully discharge and reset its memory.
Step 5: Reconnect the negative terminal and tighten it securely.
Step 6: Turn the ignition to “ON” and check if the check engine light is off. If it is, start the engine and let it run for a few minutes.
Note: Disconnecting the battery may reset other systems, including:
– Radio presets (you’ll need to re-enter them)
– Clock time
– Power window settings (on some models)
– Trip computer data
This is normal and not a cause for concern.
Method 2: Manual Code Clearing via Diagnostic Connector
Some Volvo 940 models allow you to clear codes without disconnecting the battery. This method uses the diagnostic connector to send a reset signal to the ECU.
Step 1: Turn the ignition to “OFF.”
Step 2: Locate the diagnostic connector under the dashboard.
Step 3: Use a jumper wire to connect pin A to pin B (same as when reading codes).
Step 4: Turn the ignition to “ON” (do not start the engine).
Step 5: Wait for the check engine light to flash. After a few seconds, disconnect the jumper wire.
Step 6: Turn the ignition off, then back on. The light should be off.
This method doesn’t always work on all 940 models, but it’s worth trying if you want to avoid disconnecting the battery.
Method 3: Use an OBD-I Scanner to Clear Codes
If you used a scanner to read the codes, you can often use the same tool to clear them.
Step 1: Plug the scanner into the diagnostic port.
Step 2: Turn the ignition to “ON.”
Step 3: Follow the scanner’s menu to select “Clear Codes” or “Reset ECU.”
Step 4: Confirm the action. The scanner will send a signal to erase the stored trouble codes.
Step 5: Turn the ignition off and back on. The check engine light should be off.
This is the cleanest method, as it doesn’t affect other vehicle systems like the radio or clock.
Step 4: Test Drive and Monitor
After resetting the light, take your Volvo 940 for a short test drive—about 10 to 15 minutes. Drive under normal conditions: city streets, highway speeds, and include some stops and starts.
This allows the ECU to relearn fuel trims and monitor sensor inputs. If the problem was truly fixed, the check engine light should stay off.
However, if the light comes back on within a few days, it means the issue wasn’t fully resolved. The ECU has detected the same (or a new) fault.
In that case:
– Re-read the codes
– Double-check your repairs
– Consider consulting a Volvo specialist or mechanic
Don’t ignore a returning check engine light—it could indicate a more serious problem, like a failing catalytic converter or internal engine issue.
Troubleshooting Common Issues
Even with the best intentions, things don’t always go as planned. Here are some common problems and how to fix them.
Check Engine Light Won’t Turn Off After Reset
If the light comes back on immediately or within a few miles, the problem wasn’t fixed. Possible causes:
– Incomplete repair (e.g., oxygen sensor not fully seated)
– Multiple faults (one code masked another)
– Intermittent issue (like a loose wire)
Solution: Re-diagnose with a scanner or manual method. Check for additional codes.
Battery Disconnect Didn’t Work
Sometimes, simply disconnecting the battery isn’t enough to reset the ECU. This can happen if:
– The battery was only disconnected for a few seconds
– The ECU has a backup power source
– The fault is still present and re-triggers the light
Solution: Wait longer (15–30 minutes), ensure all accessories are off, and try again. Or use a scanner.
Scanner Won’t Connect or Clear Codes
Not all OBD-I scanners work with Volvo 940. If your scanner fails to connect:
– Confirm it supports Volvo OBD-I (not just generic OBD-I)
– Check the diagnostic connector for dirt or corrosion
– Try a different scanner or manual method
Radio or Clock Reset After Battery Disconnect
This is normal. To avoid frustration:
– Write down your radio presets before disconnecting
– Use a memory saver device (plugs into the cigarette lighter) to preserve settings
Preventing Future Check Engine Light Issues
The best way to avoid resetting the check engine light is to prevent it from coming on in the first place. Here are some maintenance tips for your Volvo 940:
- Replace spark plugs and wires every 30,000–50,000 miles. Worn plugs cause misfires and trigger codes.
- Inspect and clean the MAF sensor annually. Use MAF cleaner—never touch the delicate wires inside.
- Check the gas cap regularly. A cracked or loose cap can cause evaporative emissions leaks.
- Use high-quality fuel and additives. This keeps the fuel system clean and reduces carbon buildup.
- Monitor engine temperature. Overheating can damage sensors and trigger warnings.
- Listen for unusual sounds. Rattling, pinging, or rough idling can signal early problems.
Regular maintenance not only prevents check engine lights but also extends the life of your Volvo 940.
Final Thoughts
Resetting the check engine light on a Volvo 940 isn’t rocket science—but it does require patience, the right tools, and a methodical approach. Always start with diagnosis, fix the problem, then reset the light using one of the methods outlined above.
Whether you choose to disconnect the battery, use a jumper wire, or invest in an OBD-I scanner, the key is to address the root cause. Ignoring the light can lead to reduced fuel economy, poor performance, or even engine damage.
Your Volvo 940 is a classic for a reason—it’s reliable, durable, and built to last. With proper care and attention, it can keep running strong for many more years.
So the next time that check engine light pops up, don’t panic. Grab your tools, follow this guide, and take control of your car’s health.
Drive safe, and enjoy the ride.