It will not run without that input.
The connectors/wires/plugs get old and dodgey. 
Very often they're weak.
I do not consider those sensors to have a 'wears out life span' .......I
carry a spare, but I would never change a properly working one.
You could undo the ecu plug and measure the resistance the ecu sees, all the
way through the harness to the connector, to the sensor.
Could be an open somewhere.
It always helps if you are saying air cooled vanagon, or 1.9 wbxr or 2.1
wbxr.
I assume it's a 1.9 or 2.1 
....actually, my hunch is it's a 1.9. 
very common to mess with the temp sensors on those and far less common on
2.1's. 
scott
www.turbovans.com 

when people want help with their vw vans, it sure helps to say year,
version, equipment, engine type etc,   and if selling something , where the
blank it is located. 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
James Reed-Jones
Sent: Tuesday, January 22, 2008 1:50 PM
To: vanagon@GERRY.VANAGON.COM
Subject: No start after Temp II replacement

Hey all,

So after returning from my first long distance trip (Ontario to New
Mexico and return) and learning the hard way that parts that are easy
to fix at home are much more difficult to fix when stuck in Nebraska
on a weekend I decided to start preventably changing out a few small
items in my 87 westy.

Today was the temp II sensors turn (to see if I could get rid of some
other issues that cropped up during the voyage).

So no problem with the replacement (I thought) until I tried to start
the engine. It will turn over but won't catch. I have tried looking to
see if I knocked something loose but couldn't see anything, I also
have tried starting with the sensor unplugged, plugged in, and plugged
into the old sensor (dangling - I didn't swap it back).

Any suggestions as to what I may have done and can look for would be great!

Thanks,

James