I have a 2011 Chevy HHR. My coolant reservoir has a leak in it. I went to get my oil changed and I asked the attendant to cap off my coolant reservoir because I saw fluctuations on it. He did and it was empty. He put some in and you can see it leaking. The engine runs fine if there is coolant. Also my check engine light has not come on. Is replacing the reservoir the only needs doing?
From your description, yes, that's probably all you need to do. If you can absolutely verify the reservoir is what is leaking, as in, see exactly what is leaking, I'd think you're good. I'd first verify why it's leaking. Is there a crack or chip out of the reservoir? Is it leaking where the tube attaches to it (very common leak source for reservoirs)? If so, you've found your culprit. While you are there, I'd double check the tubing is good (most likely should be). You'd be looking for dry rot (cracking). You may also need to replace the hose clamps if they are weak (not likely). Other than that, yah, just replace the reservoir and most likely you're golden.