Hi there!
I have an iPhone 3G with jailbroken iOS 4.2.1 on it. I read an older article and installed 06.15.00 iPad firmware in order to unlock it so I can use it for what it should be (a phone). It had major battery drain issues and GPS was not working (the default problems), so I searched around some more and found out that the baseband can actually be downgraded from 06.15.00 to 05.13.04 now, so I went at it. The first try did not succeed and I was still on 06.15.00 after trying to downgrade it with redsn0w 0.9.14b1, I reinstalled the 06.15.00 baseband and tried downgrading it again with the newest redsn0w 0.9.14b2 version. This time I succeeded.
However, now in the information section by Modem Firmware it was completely blank, but I knew that this time it downgraded, since it didn't show 06.15.00 anymore, so that counts for something, right?
Here is the problem though, now I get no signal, and my phone says no wi-fi also. I have read that the no signal problem can be fixed by reinstalling ultrasn0w and Mobile Substrate, but I can't do that with both no signal and no wi-fi.
What are my options here?

Try restoring to a regular 4.2.1 firmware to see if that puts your baseband back on there. Right now it sounds like your baseband is corrupt.

If the BB is corrupt, I hope you can get it back, if you really manage to fry it it's possible you can throw the phone away or use it as an expensive paper weight.

Holy shit! Restored to a clean 4.2.1 firmware, jailbraked it with redsn0w and now it's fixed!
05.15.04 firmware, everything is working. Thanks!
Where do I go from here, do I need to downgrade 05.15.04 to 05.13.04, or is there no need for that?

If you don't want to unlock it just leave it as is. If you still need to unlock you will need to update to 6.15 baseband then downgrade to 5.13. Do you know what bootloader you have? If not you can find out with f0recast. You should find that out first before doing anything.

The bootloader is 6.4_M3S2, I searched around and it looks like only 5.8 can be downgraded to 05.13.04
Fucking bullshit. This means I have to install 06.15.00 baseband again, and downgrading will most likely end up with corrupted BB once again.
And I do need it unlocked, otherwise it's useless.
God damn.

That's old info. Redsnow 0.9.14b2 can downgrade 6.15 to 5.13 now on any 3G or 3GS. Sounds like something just went wrong the first time you tried it.

It seems like not every 3G can be downgraded apparently. I have gone through this twice, and both times it ended up with corrupt baseband, no wifi, no bluetooth, no imei and no service.
The process goes through solid every time, but it ends up corrupt. Could it be some rare piece of hardware that still can't be downgraded?

Might be, MuscleNerd probably wasn't able to test it on every single hardware revision of 3G and 3GS's. If that's the case and you need an unlock you can either update to the ipad baseband again and just use that. Or if it is an ATT iPhone ATT should be able to unlock if for you at this point.

If you are feeling adventurous you may try and downgrade the baseband again and when you are at the point where the baseband is missing try restoring to 3.1.3 instead of 4.2.1 to see if it loads that baseband (5.12 I think it was). That baseband can be unlocked.

Can't really use 06.15.00 baseband, it drains the battery and uses my cellular network all the time looking for push certificates, and that can't be fixed since Apple blocked SAM a few months ago. The phone overheats and becomes unusable eventually.
I tried contacting MuscleNerd on Facebook about this, but had no response. Know any other ways to contacting him?
Official activation is sadly not an option.

Alright, I downgraded and I am at the point with no BB again. What FW should I try restoring?
If I restore to 3.1.3 with 5.12, will the baseband be preserved to 4.2.1? I was on 3.1.3 before with 06.15.00, it ran perfectly, but could literally run none of the new apps in the market today. So I would not like to stay on 3.1.3 again.
Maybe I should try 4.1? iOS 4 appearantly has 05.13.04 as the default baseband, but could it really be updated on the phone in this state?