Is it safe to leave my old gas line behind an electric oven?

You definitly shouldn't just leave that yellow flexible line hanging back there, and you definitely don't need to mess with any silicone sleeves. The heat from an electric range is significant, but gas ovens get plenty hot too, so the pipe itself can handle the temperture. The real issue is that those flexible connectors are designed for a permanent hookup, not for sitting loose behind an appliance where they can get pinched or damaged by the back of the new unit.

The safest move without spending a fortune on a plumber is to remove that yellow flex line entirely. You'll be left with the shut-off valve sticking out of the floor. Go grab a **brass cap**—ussually 1/2" or 5/8" flare depending on your specific valve—and some yellow gas-rated Teflon tape. Wrap the threads, screw the cap on tight, and then do a leak test. It's way better than just trusting an old valve handle not to get bumped.

To test it, just mix some dish soap and water and wipe it over the capped connection. If you see bubbles forming, you've got a leak and need to tighten it or re-tape it. Once it's solid, leave the valve in the "off" position. This gives you two layers of protection: the valve and the physical cap. It's much safer than just crossing your fingers while you're shoving a heavy stove into place.

One thing to keep in mind is the clearance. Electric ranges have that massive 4-prong plug and the cord needs space to fold up. If that gas pipe is sticking up too high, it might prevent you from pushing the stove flush against the wall. If it’s in the way, that’s when you’d realy want a plumber to go into the crawlspace or basement to cap it off below the floorboards and remove the stub entirely. It'll look cleaner and let the oven sit where it's acutally supposed to.

That leak test with the soapy water sounds easy enough, but does it matter what kind of soap I use? I've got some heavy-duty degreaser in the kitchen but I'm worried it might be too harsh for the metal threads. Also, about that yellow tape—is that actually diferent from the white stuff plumbers use for sinks? I have a roll of the white tape in my junk drawer, but I'm guessing gas needs something specific? I relly want to make sure I don't mess this up since I'm trying to do this myself without calling a pro for every little thing.

Don't use that degreaser. Most of those have ammonia, wich can actualy cause stress corrosion cracking on brass fittings over time. Just use regular blue Dawn or any mild dish soap. As for the tape, the yellow stuff is much thicker and specifically rated for hydrocarbons. White tape is too thin and can shred or dissolve if it comes into contact with certain gas additives. Also, check your new range’s installation manual for the 'utility zone' diagram. These stoves usually have a recessed area in the back frame. If your gas stub is outside that zone, you’re never getting that oven flush to the wall, even with a cap. You might end up needing to move the pipe anyway just for the fit.

Make sure you pull up the installation manual for that specific model. Most of them have a diagram showing the 'void' area on the back. If your gas stub is even an inch outside that zone, the oven won't sit flush against the wall, wich looks terrible and makes the anti-tip bracket impossible to install. I learned that the hard way with a range where the back was almost entirely flat. If it’s in the way, don't try to force it or you'll just end up kinking the new power cord or damaging the valve.

If you keep the stub-out, watch out for leverage. These electric ranges are heavy, and if the back panel catches on that valve while you're pushing, you can put a ton of torque on the pipe. I've seen folks loosen a joint below the floorboards just by shoving the unit back too hard.

Also, make sure your new 240V cord isn't resting directly against the capped pipe. Vibration from the convection fan can cause the cord insulation to rub against the metal over time. I ussually zip-tie the slack to the back of the range to keep it clear of the plumbing.