The check engine light is a general warning that something is wrong with the drivetrain. The problem behind it may be serious, but doesn’t have to be, especially if the light goes off by itself. In some cases, you don’t even have to replace anything and it all comes down to misuse.
If the check engine light goes off after a week, there is a chance that you left the gas cap loose. It could also be that it’s a random freeze-frame fault like a misfire that doesn’t indicate anything serious. But there is also a chance that one of the sensors is faulty and is going in and out of voltage range causing the check engine light to turn on intermittently.
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How Does the OBD Diagnostic System Work?
The OBD system is connected to every sensor inside the vehicle. And each of those sensors has a feedback or signal wire going back to the engine control unit. The way that works is that each sensor operates within a certain voltage range, which is usually 0.2 to 2 volts. As an example, we will take the coolant temperature sensor. The hotter the coolant, the lower the sensor resistance. And the lower the resistance, the higher the voltage feedback is.
When the coolant is cold, the sensor sends 0.2 volts to the ECU, and when it’s at 90 degrees, it sends 1 volt. And the only way the OBD system can know that the sensor is faulty is if there is no signal coming back to it, or if the signal is out of range. That means if the signal is over 1.5 volts which the engine control unit cannot interpret.
Once a sensor goes out of voltage range, the engine control unit triggers the check engine light and a corresponding fault code. There are also other things that trigger the check engine light like misfires which don’t have a sensor, or vacuum leaks which also don’t have sensors. In such cases, the fault code will tell you what the sensors detected, but it can’t tell you the reason behind it.
And there is the reason why the check engine light comes on and off by itself. The check engine light comes on when a sensor is out of range, and turns off when it comes back into the voltage range. Vacuum leaks and misfires are also intermittent in most cases which is why the check engine light comes on and off randomly. An important thing to note here is that even after the check engine light goes off, the fault codes will remain stored.
Why the Check Engine Light Went Off
1. Faulty Sensor
Some of the sensors in a car are more important than others. If the engine control unit notices a certain sensor is out of range, it will trigger the check engine light and disable that sensor. The engine control unit then checks the voltage of that sensor each time you turn the engine on.
And if the sensor comes back into range and remains within range for a specific time period, the engine control unit will turn it back on again, and turn off the check engine light. But that doesn’t mean the affected sensor is now working properly. If it went out of range even once, it means it’s faulty. And now that it’s back in range, it’s most likely giving the engine control unit false information.
When that happens, the engine won’t run properly, and the check engine light can come back on for a different reason. Depending on what sensor is faulty, those can be rich or lean fault codes, misfires, timing belt/chain out of sync, and so on.
The problem is, that the engine control unit can’t detect false information, it can only detect a dead sensor or when a sensor is out of range. The point is, if the check engine light came on once, you should still have the vehicle inspected to make sure the code wasn’t triggered by a faulty sensor.
2. Loose Gas Cap
Yes, a loose gas cap can trigger the check engine light. The gas cap plays a very important role in the EVAP system which is present in pretty much all cars made after the nineties. EVAP is short for evaporation and it’s a system that diverts gasoline fuel vapors from the gas tank to the engine. That prevents the fuel vapors from going into the atmosphere and polluting the environment, plus it saves you some gas.
The system relies on fuel tank pressure to know when to send the fuel vapors into the charcoal canister near the engine. But if the gas cap is loose, the fuel vapors won’t be able to build pressure and there won’t be any vacuum when the purge valve opens. To detect this, there is a pressure sensor inside the fuel tank, and as soon as it notices the fuel tank pressure isn’t where it needs to be, it triggers the check engine light.
The fault code you will get with this problem is P0457. To fix but also prevent it from happening in the future, you want to tighten the gas cap as much as you can until it starts to click. The clicking might sound like you are breaking the gas cap, but don’t worry, it’s meant to do that.
And if this doesn’t fix the problem, there is always a chance that the gas cap is faulty and is leaking even when you tighten it properly. But to diagnose that you will need to scan the fault codes and buy a new gas cap only if get the P0457 fault code.
3. Random Freeze Frame Faults
There are two types of fault codes the engine control unit detects. The first is a fault code that’s triggered by a constant issue like a dead sensor. The second type is a freeze frame fault. A freeze frame fault means it happened only once and it’s most often triggered by a misfire. Misfires are normal to some degree and if you haven’t noticed any other symptoms other than the check engine light going and off, there is no need to replace anything.
A misfire can happen when you drive through a deep puddle and one of the spark plugs gets wet. It can also happen when you drive too slow in a certain gear, or there could just be a random ECU glitch that causes a misfire. But whatever the case, you still have to scan the fault codes first to see what’s happening. It’s almost impossible to notice a random misfire, and it’s impossible to notice a slightly off air-fuel mixture.
Will the Check Engine Light Come Back On
Whether or not the check engine light will come back on depends on what’s caused it in the first place. If the reason behind the check engine light was a failing sensor, then it almost certainly will come back on. When that will happen is difficult to tell. Just recently it happened with my car and the check engine light would only come on after a few hundred miles of driving.
Once I turn the engine off and back on, the check engine light goes out. But as long as I was driving around town, the check engine light never came on. That means it wouldn’t come on for weeks until I go on a road trip, and it turns off as soon as I come back.
And the reason behind it was a failing coolant temperature sensor. I did notice that the car’s performance was reduced slightly regardless of the check engine light, which tells you that the sensor was sending false information the entire time.
But if the check engine light was caused by a loose gas cap or a freeze frame fault, it won’t turn back on unless you replicate the same circumstances that triggered it the first time. Again, that could be driving through a puddle a little too fast, leaving the gas cap loose, or it’s just down to bad driving.
What Should You Do About the Check Engine Light
Even after the check engine light turns off by itself, the fault codes will remain stored in the engine control unit for a long time. And since the problem behind it could be permanent, it’s a must to scan the fault codes and see what it’s about. Scanning the fault codes is pretty simple. You can buy an OBD2 scanner or an OBD2 Bluetooth adapter.
You can find both for less than twenty bucks on Amazon, but keep in mind that you have to download an app on your smartphone if you go with the Bluetooth adapter. But both will scan the fault codes and you can delete fault codes with both. If you want someone else to do it for you, you can have the fault codes scanned at any AutoZone or NAPA store for free. Doing so at an independent shop or a dealership will likely cost about $30.
Once you have the fault codes, you can find all of them on Google, and if you are not sure about the interpretation, visit some online forums. If that doesn’t help, I’m afraid you will have to visit a repair shop. But when the check engine light comes on and off randomly, the fault code is pretty straightforward and doesn’t leave much room for interpretation.
Final Words
Ultimately, even if the check engine light went off by itself, rest assured that something was wrong and you should scan the fault codes. The only thing you can do before that is try and remember if you refueled on the day the check engine light went off, or maybe the day before.
In that case, chances are the fuel cap was left loose the last time you got gas. But if that’s not it, chances are one of the sensors is faulty and is going in and out of voltage range as you drive. That causes the check engine light to come on and off intermittently.
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