It’s a common scenario for BMW 335i owners: you get the dreaded “low key battery” warning on your dashboard. Thinking it’s a simple fix, you decide to replace the battery in your 2009 Bmw 335i Key Fob. However, sometimes things don’t go as planned, and you might find yourself with a key fob that no longer operates the remote functions of your car after a battery replacement. You might be able to start your BMW, but the key fob won’t lock or unlock the doors, or open the trunk.
If you’re facing this issue with your 2009 BMW 335i key fob after a battery change, don’t panic. Let’s troubleshoot some common causes and potential solutions.
One of the first things to verify is the battery itself. Even if a new battery is advertised as 3V, it’s wise to test it with a multimeter to ensure it’s actually providing the correct voltage. A seemingly new battery could be faulty or have lost some charge. You should be seeing around 3 Volts.
Another critical aspect, especially with older key fobs like those for a 2009 BMW 335i, is the soldering. Many DIY guides, and likely the video you watched, suggest desoldering the old VL2020 or ML2020 Panasonic battery and soldering in a new one. If you performed this yourself, carefully inspect your solder joints. A poor solder connection can prevent the new battery from properly powering the key fob’s circuitry. Ensure the solder points are clean, solid, and making good contact. If you’re unsure, reheating the existing solder points might resolve a cold joint issue.
It’s also worth considering if any damage occurred during the battery replacement process. While replacing the battery, it’s possible to inadvertently damage delicate components inside the 2009 BMW 335i key fob, especially if force is used when opening the case or handling the internal circuit board. Carefully examine the circuit board for any signs of cracks, bent components, or dislodged parts.
Even if you revert to the original battery, if the key fob’s internal circuitry was disrupted during the process, it might no longer function correctly. It’s less likely that replacing the shell case itself would cause this issue unless it somehow applied pressure to the internal components, but it’s worth considering if the new case is fitting too tightly. Try using the key fob without the case to eliminate this possibility, as you’ve already attempted.
If you’ve checked the battery voltage, inspected your soldering, and ruled out physical damage, and your 2009 BMW 335i key fob still isn’t working for remote functions (but starts the car), the next step would be to seek professional assistance. While battery replacement is often straightforward, intricate issues within the key fob’s electronics may require specialized diagnostic tools and expertise. A qualified automotive locksmith or a BMW dealership service center can further diagnose the problem and offer solutions, which might range from key fob reprogramming (though less common after just a battery change) to key fob replacement if necessary.
