All of these issues are covered in the relevant console page (most with more info) but this is just a quick document to detail the most common issues.
This section only really applies to OEM Game Paks and has little bearing on bootleg or aftermarket hardware. Check out this visual guide or even the /r/gameverifying wiki to find out how to spot bootleg hardware.
How to replace a battery in a Game Boy cartridge. If you use the "tape method" you will be shunned and made fun of, simultaneously. Solder and a tabbed CR1616 (must for GBA games) or tabbed CR2025/CR2032 are required.
The battery is not required for saving but instead for the Real-Time Clock (RTC) hardware. Without this battery, certain in-game events will not work such as Shoal Cave tides or berry growing. If this battery is dead, upon starting the game, you'll get a message that "The internal battery has run dry." but everything else (aside from RTC based events) should still work normally, including saves.
Due to a quirk with the initilization of the RTC hardware, if you replace the battery in one of these games without deleting your save, your RTC still may not work despite the error message no longer occuring on the main menu. While not the exact same issue, the symptoms are identical to the berry glitch from Ruby and Sapphire (but it affects Emerald as well) but the fix is different. There are several different ways to fix this:
This problem more commonly affects original Game Boy (Color) carts but may also affect Game Boy Advance carts as well.
When starting a Game Boy console with a game inserted, the console freezes on the boot screen with a garbled or incomplete Nintendo logo. Another manifestation can be if the cart does boot but seems to freeze on startup or intermittently. As a form of copy protection, the actual "Nintendo" logo is stored within the ROM of the game and read by the Game Boy when booting and compared to a copy stored internally within the Game Boy. If the console cannot read this logo, it does not boot the game.
You may have heard of the below as potential fixes for carts not reading, but they are a bad idea in the long run:
Check where the problem lies:
In and out: Simply remove and reinsert the game. If this does not help, the most common cause is dirty pin on either the cart, the console, or both. If cleaning the cart and the console does not resolve the issue, another common cause may be broken solder joints.
BOBdotEXE sums up these troubleshooting steps nicely.
Disassemble: The cart may be cleaned without disassembly but disassembly does make the job easier. You may also take this opportunity to service the battery (if equipped) and to inspect for damage or corrosion. Recommended cleaning method is isopropyl alcohol on a cotton swab. Gently rub an isopropyl alcohol saturated cotton swab along the pins. Repeat with a fresh swab and more isopropyl alcohol until the swab comes away from the pins clean.
Artist eraser: If you can disassemble your cart, a better option for cleaning the pins is to use an artist eraser. Simply rub the eraser along the pins until they appear cleaned. Please see the above video for an example.
Cleaning kits: The cart reader may be cleaned with the official Game Boy cleaning kit but since you likely don't have one of those (not gatekeeping, they just didn't sell well and have been long since discontinued), there are also third party options available. Another good option is to use a credit card sized object wrapped in a cotton t-shirt or similar thickness lint free cloth and use that to clean the cart slot. Isopropyl alcohol may help this process, same as above. If using this method, always move in and out and never side to side else you risk damaging the pins.
Inspect for damage: Visible damage or corrosion to the cartridge pins is a bad sign. It may be indicative of other issues but will need to be dealt with before moving on. If a gold finger is broken or disconnected, unfortunately, it's game over for that cart. You'll need to replace the PCB with a suitable donor, or, if applicable, a new PCB. As long as the contacts are only damaged or dirty on the surface, they can be restored.
Broken solder joints: Game Boy game PCBs are very thin at 0.9mm thick and, unfortunately, very flexible. Over the years, inserting and removing the cartridge will cause the PCB to flex a little and this will put stress on the solder joints. Eventually, this stress will lead to fractures and these fractures are just small enough to not be visible to the naked eye but big enough to cause electrical connectivity issues. While annoying, the solution is rather simple, albeit difficult if you're not experienced with soldering. You'll need to redo the solder on the pins of the MROM chip (the one on the lower right in most Game Boy cartridges) and potentially the MBC chip as well (the top left in most Game Boy cartridges). Removing the battery, if equipped, while resoldering is highly recommended. See the above linked video for an example on redoing the solder joints in the cart. Using flux will yield significantly better results.
Light abrasives: Light surface damage, corrosion, or other gunk that cannot be cleaned with isopropyl alcohol and a cotton swab can still be dealt with but will require more extreme measures. Sometimes, the contacts will need a good scrub but if that absolutely does not work, another option is a mild abrasive. This is only for visible issues. If the pins are visibly clean, do not polish them. Some people have reported success with Brasso or other surface polishing compounds, other people use a baking soda paste. When used very liberally, compounds like this generally will take off anything built up on the surface layer of the contacts without actually damaging the contacts. When using this, you only want to use up and down motions to polish, not side to side as this may clean the pins unevenly. Make sure you clean up afterwords with isopropyl alcohol or whatever your preferred cleaning solution.
The console's power LED flickers, low battery light comes on early or intermittently, or the console randomly resets are all issues caused by a dirty or faulty power switch. Dirty or damaged switches can usually be fixed by cleaning out the switch.
The most effective method to clean the power switch is to disassemble the switch and ensure everything is clean all the oxidation and buildup from the inside-out. The far less effective method to clean the switch is to spray contact cleaner or isopropyl alcohol inside the switch and simply work the switch back and forth. Make sure to let the switch dry out before turning it back on. Opening the switch is the most reliable and best long term solution. Other methods may still help, but should be considered a temporary fix.
AGBs and AGSs exhibit a flickering lower power LED and are the simplest to clean. Video
CGBs and MGBs share a similar physical switch body and are opened the same way. Video They do have a latch that the AGB/AGS/OXY do not have and will require extra care. Video
OXYs can be cleaned as well. Video
On an unmodded console, the most common issue is a dirty or damaged power switch and can usually be fixed by cleaning out the switch. See above.
Blown fuses are a bit harder to fix simply because of the tiny soldering required. To test your fuse, a multimeter is usually needed. Simply measuring continuity across the fuse will tell you if the fuse has blown. Continuity shows power is flowing and the fuse is good. No continuity means the fuse has blown and opened circuit. Most Game Boys have two fuses, usually marked “F1” and “F2” on the main boards. Usually one fuse is connected to the battery terminals and the other fuse connected to the external power input, ie charging port in AGS and OXY or DC jack on CGB and MGB. Note, fuses are safety devices and a blown fuse is cause for further investigation and diagnostics. Please see this (WIP) list of Game Boy components to find the proper replacement fuse.
Damaged DC power jack in a Game Boy Pocket or Game Boy Color system can make the console appear dead. A simple USB to DC power adapter can help diagnose this issue. Similarly to the headphone jack, a physical switch exists in the DC jack that disconnects the battery terminals from the system when on DC power. Contacts within the DC jack may corrode from moisture and open the physical switch permanently. Careful cleaning can help, but with the center pin in the way, thorough cleaning may be difficult. Limited new and used DC jacks are available through eBay and AliExpress. For diagnostics, pin 2 and 3 as labeled on the console may be bridged. This should not be a permanent fix without fully removing the DC jack. Applying power to batteries with a DC jack inserted is dangerous. Alkaline batteries can explode.
Bad battery or battery connector in some models can cause issues. In models that came with rechargeable batteries (AGS and OXY), the battery may need replacement. AGS OXY
On alkaline models, the connectors can get dirty or completely corroded. In cases of mild dirt or corrosion, a simple clean with a cotton swap and isopropyl will resolve the issue. In heavier cases of corrosion without physical damage, distilled vinegar and a cotton swab may be used to clean away all the corrosion. Terminals can be desoldered from the console and removed from the case shell to be soaked in vinegar for more severe corrosion. When vinegar is used, isopropyl should be used to wash away the vinegar as it is corrosive and will damage the board. In extreme cases of corrosion and battery leakage, the terminals will be corroded through and require replacement. DMG MGB MGL/CGB AGB
There are a few potential causes for these: