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Knock Sensor:  Everything You Need to Know

Knock Sensor

Engine knock is what you call a problem with pre ignition or pre detonation in the engine of your vehicle. It's s kind of misfire that happens when the combustion reaction, which is literally a small explosion inside your engine, doesn't happen when it's supposed to happen. This produces a distinctive sound and vibration from your engine.

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When your combustion reaction isn't happening exactly as it should, and the timing of your engine has been calculated and engineered down to fractions of a second, you can end up having misfires and stalls as a result. The combustion reaction should be happening at the very top of the compression stroke in the cylinder of your engine which is called top dead centre. When it starts to go bad enough thanks to mistiming, you'll end up hearing the sound caused by your engine misfiring which is what we refer to as knock.


The knock sensor in your engine exists to pick up on the sounds before they get too bad. The sensor is sensitive enough to understand when your engine is just beginning the process of suffering from poor timing that can lead to serious knock and help your car's computer make adjustments to prevent it from getting any worse. Ideally your knock sensor will be hearing knocking before your ears can pick it up. 


As a safety feature in your vehicle you want to make sure your knock sensor is working as well as it can all the time. Let's take a look at how you know you're having a problem with your knock sensor and also what you can do about it when it does go bad.


What Is the Knock Sensor For?


 The knock sensor is installed in your engine as a way to sense vibrations and sounds that come from your engine while it's running. People typically refer to this as engine knock, so that's why there's a knock sensor. When it senses significant vibrations or sounds the data is then related back to your car's computer or engine control unit. Based on the sensor data your engine is able to determine whether or not things like ignition timing need to be altered in your vehicle to keep it running smoothly. 


Your car's computer can then alter the way the spark plug works and have it spark earlier or later in order to improve the timing of the combustion reaction and reduce the problems you're having with free ignition or engine knock. 


In so many words, the knock sensor in your car is a piece of preventative safety equipment. Its job is to detect a problem before you are able to detect the problem and make adjustments to fix it before it goes from bad to worse. However, if the knock sensor itself isn't working then obviously this can't happen.


What Happens When a Knock Sensor Goes Bad?


 If your knock sensor isn't working the way it's supposed to then the little timing errors that produce small amounts of engine knock are going to go unnoticed by your car's computer. That means that a minor timing issue can grow into a much bigger timing issue because there's nothing alerting your car's computer to the fact that it needs to make adjustments. When the situation is bad enough, you're going to end up experiencing a number of problems.


Noises:  They don't call a knock sensor a knock sensor for no reason. When your knock sensor isn't working you'll start noticing the sounds that it should have been picking up on before they got too loud. These can range in the beginning from pings and thumping sounds too much louder and obtrusive noises as the problem gets worse. You'll hear a knocking sound and in some cases, it can get so loud and annoying that it sounds not unlike someone filling a can with rocks and shaking it in your engine. That's a seriously bad case of engine knock and ideally you would have taken it to a mechanic well before it got to that point, but that is what can happen if you let it go for too long.


The sounds in your engine will get louder and will last longer if you ignore them. The noises themselves are caused by the ignition reaction happening in your engine when it's not supposed to. If it happens at the wrong time and when the valves and pistons are in the wrong position, you're going to have mistimed explosions that create this cacophony of destructive sounds.


Bad Gas Mileage: When your knock sensor is working the way it's supposed to is going to maximize your fuel efficiency. However, if it's not sending the correct signals to your car's computer any longer than that's not going to happen. You'll be wasting fuel as it's sprayed at the incorrect time and not being ignited properly by your spark plug. The end result will be a poorer performance and more trips to the gas pumps as a result.


Poor Acceleration: A failed knock sensor will make it harder for your vehicle to perform at its peak. You'll notice this the most when you're trying to accelerate. Your car is going to struggle to get up to speed, even if you're putting your foot right down on the gas pedal. You'll be experiencing lower engine pressure overall, and potential misfires which all reduce the ability of your engine to work the way it's supposed to and give you the power you're expecting.


Rough Performance: A bad knock sensor can lead to your car struggling and jerking as you drive. It will feel sluggish and almost like you're dragging something, or your brakes are slightly engaged. This could all be caused by your knock sensor not working properly and your engine suffering some serious damage because of a prolonged amount of pre ignition problems.


Performance Drop: A slightly different situation and one you won't find in every vehicle occurs when your car's computer realizes the knock sensor isn't sending correct data any longer. Your car will then reduce the power of your vehicle as a safety precaution. Some people would call this going into limp mode. The car will slow down and perform much more poorly as a precaution so that you are able to get to a mechanic to have the knock sensor fixed as a result. This is meant to prevent any further damage caused by driving without a functioning knock sensor. 


Check Engine Light: A bad knock sensor is one of many, many potential causes for the check engine light to pop up on your dashboard display. Unfortunately this warning light is very vague so you'll never know for sure why it came up unless you're experiencing some of the other symptoms we mentioned and you're able to get your car into a mechanic. Ultimately, you can use an OBD2 scanner to try to diagnose the check engine light problem yourself.


 An OBD2 scanner, or on-board diagnostic tool, is the same diagnostic tool that a mechanic will use to figure out why the check engine light came up on your dashboard. When you plug it into your car it gives you some additional information about the check engine light including a code that relates to a specific area of your engine. In this case, it will throw a code letting you know that there's a problem with your knock sensor. When you have one of these tools on your own, which you can buy for only $30 or $40 on amazon.com, you can save a lot of time diagnosing your problems. 


 Can a Bad Knock Sensor Damage the Engine?


As we have seen there are some serious side effects that can potentially occur when your knock sensor isn't working the way it's supposed to. If your car is allowed to continue misfiring on a long enough timeline then you could definitely suffer some severe engine damage as a result. Damage to the valves and pistons as a result of misfiring can lead to warping, you could have blown head gaskets and more. You can potentially be looking at several thousand dollars worth of repairs to your engine which started initially with a knock sensor failing on you. It's kind of a cascade effect, as the knock sensor goes bad and then more things start failing as a result of the knock sensor being unable to keep everything running smoothly.


 How Much Does It Cost to Replace a Knock Sensor?


It's in your best interest to keep the knock sensor in your vehicle working properly, you probably want to know what you can do about it when it does go bad. Repairing a knock sensor is much cheaper than repairing the potential damage that can be caused if it's allowed to malfunction for too long. A typical knock sensor replacement is going to cost you somewhere between $120 and $500. That includes the cost of labor but of course it will vary depending on the make and model of your vehicle, and the mechanic that you take it to.


If you're just interested in the part itself, you can head to AutoZone and pick up a new knock sensor from anywhere between about $20 and $75. There are some very high-end knock sensors that you can pick up as well that have a pretty hefty price tag of between $200 and $370, but it's not likely that you would need something that expensive. It's best to check with your owner's manual and make sure you know exactly what kind of sensor your vehicle needs and then price a part accordingly. 


It's also possible you can save yourself some money on the labor costs by handling the job yourself. Obviously, this isn't the kind of thing most of us have done before, but thankfully there are a number of videos that you can find on YouTube that can walk you through the process. The most difficult part is just figuring out exactly where the knock sensor is in your particular vehicle as it can change from one model for the next fairly significantly. You may need to do a bit of hunting to find a video that will show you the exact process for your specific make and model if you're not very comfortable working under the hood of your car yet. 


However, videos like this one give you a pretty clear demonstration of what needs to be done with important details included like what you should be doing with electrical harnesses when you remove them and so on. 


Can I Drive Without a Knock Sensor?


Technically speaking you can in fact drive around without the knock sensor in your engine. Does that make it a good idea? Not particularly. The knock sensor is an important safety feature of your vehicle, and if you don't have one properly installed then you're just putting yourself at greater risk for accidents as well as bigger repair bills down the road if something were to go wrong.


As you've seen, all the symptoms of a bad knock sensor are the exact same things that will happen when you have no knock sensor installed. They will continue to get worse until the damage to your engine becomes significant. It's best not to even risk the trouble and make sure you get your knock sensor fixed when you can.


The Bottom Line


Even though many drivers are unaware that their car has a knock sensor in the engine or what it does, it's definitely an important part of maintaining the optimal performance of your engine. You can think of it like something that helps you out by listening for problems, so you don't have to worry about it. But you do need to worry if it's not doing its job any longer. For that reason, when you do notice any of the symptoms that we mentioned of a bad knock sensor, you're going to want to head to a mechanic as soon as you can so they can replace the part for you if you're not comfortable handling that kind of repair work on your own.


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