Join Date: Jan 2011
Location: Denton TX
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Go ahead and replace the hose while you're at it. When hoses get old, they either get really brittle or really soft (radiator hoses usually get real soft). You already said it's 11 years old; that's far, far beyond the suggested lifespan. A new hose is cheap and takes all of 5 minutes to replace, though you do have to drain some coolant out first. Don't reuse the original clamps, they're a pain in the ass.
You're correct in that the suction will affect the lower hose only, at least on this car.
The sensor on the upper hose is the sending unit for the temp gauge on the dash - it's not connected to the ECU in any way. Disconnecting it will only result in your temp gauge sitting on "C" all the time. There's another temp sensor (I want to say on the back of the engine?) that tells the computer how hot the engine is; if you're still getting that code after you fix the collapsing hose, then that sensor may be bad or disconnected.
If the upper sensor (on the upper hose) is bad, a new one is cheap; a brand new OEM one was $15 at the dealer when I replaced mine, and AutoZone has them for about $10.
As FreakFlier suggested, replace the cap and make sure the line from the radiator to the overflow is clear. Bleeding the cooling system is a bit of a pain as well.