How To Fix P0304 Code For BMW Vehicles (Misfire Cylinder 4

Do you know how to fix the P0304 code for a BMW vehicle? this is one of the questions our readers ask a lot. Well, we´ve got you covered.

Diagnostic codes are your vehicle’s way of letting you know that something is wrong. Owning a diagnostic tool will allow you to get the exact code hidden behind the check engine light and get a head start on getting your BMW back to peak performance.

The p0304 code for all BMW vehicles means that there is a misfire detected in cylinder number 4. Anytime there is a misfire in any cylinder, the BMW’s computer will notify you with a check engine light and there will likely be some accompanying symptoms.

There are a number of issues that can cause a cylinder to misfire, so properly diagnosing it means getting to the source of what is causing the misfire.

The first thing you’ll want to do is figure out why the check engine light came on.

Table of Contents

Diagnosing A Cylinder 4 Misfire

Unfortunately, most vehicles only display a check engine light when something goes wrong.

To discover the problem, start by getting the code with a diagnostic tool or by taking it to your local mechanic.

Since most mechanics will charge you anywhere between $20 and $50 just to run a diagnostic tool on your BMW, purchasing your own diagnostic tool is a great way to save money, figure out what is wrong, and help you plan ahead.

You can pick up an OBD2 scanner—such as this one— at almost any local retail store. With the cheaper ones, all you will get is the code, such as p0304.

More expensive OBD2 scanners will provide you with an in-depth explanation of the problem.

Symptoms Of A Cylinder 4 Misfire

If it’s a one-time thing or something that happens very infrequently, you may not experience symptoms at all. A repeated misfire will definitely grab your attention, however.

A solid or flashing Check Engine Light. A flashing check engine light means you’re likely to break down any second.
Poor acceleration, jerking, stalling, or unevenness while cruising at normal speeds.
Rough idling. BMW stalls at stoplights or stop signs.

Most likely, there won’t be any symptoms outside of the check engine light, however, a check engine light is always cause for a check, even if you don’t feel like anything is wrong.

Since most of the above-listed symptoms may not appear at all, a check engine light is your only indication that something is wrong and you should either check it with your own diagnostic tool or take it to a mechanic.

A flashing check engine light is problematic immediately. It means something is seriously wrong, like cylinder four misfiring constantly, which also means there’s a more serious, underlying issue at work.

Potential Causes For A Cylinder 4 Misfire

The potential causes are numerous and will probably be difficult to narrow down. If you own an OBD2 scanner that is heavy on detail, this may be an easier process.

If not, it will necessitate a process of elimination on your part, especially if you’re looking to avoid the mechanic.

Spark plugs, distributor cap, or ignition coils are bad/failing
The timing chain is off
Fuel pressure is low
Crankshaft or Camshaft sensors need replacing
The throttle position sensor is bad
Valve issues or leaking head gasket

The timing chain, leaking head gasket, and fuel pressure possibilities are the worst-case scenario problems.

Unless you’re a mechanic with a lot of knowledge and all of the proper tools and equipment, none of those are issues you’ll want to tackle at home.

Estimated Cost Of Repair

The standard cost of repair for a misfiring cylinder number four in a BMW is $125 before labor.

However, if some of the above issues are the cause of the misfire—such as the timing chain, head gasket, or valves—you’re potentially looking at thousands of dollars in labor alone.

Timing chains are especially brutal, given the time (in terms of labor) that it takes to get to them. Though timing chains themselves aren’t particularly expensive—of course, that’s a matter of perspective—the sheer lunacy of labor costs is what will get you.

Valves and head gaskets are potential problems (as far as price is concerned) as well.

Of course, all of this is potentially DIY work but it will be extremely time-consuming and labor-intensive if you’re alone and performing the work with a limited toolset.

Your best bet is to check around town for the best price on repairs or to hope that it’s something far simpler, such as a sensor, spark plug, or ignition coil, any of which is generally easy to replace and inexpensive,

All In All

The p0304 code isn’t necessarily causing serious alarm, but if you’re getting that code from your BMW, it’s important to check it out, either yourself or by a mechanic.

While it could be something as simple as a spark plug, even a simple problem can quickly grow into a big one.

In any case, never let a check engine light go unanswered, as the potential fallout can be devastating to both your bank account and peace of mind.

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