Sounds very similar to a problem I sorted out last weekend - car wouldn't start unless the wiring to all 4 injectors was unplugged - it was the wiring to the water temp sensor that was at fault. To check yours is OK, take the back off the ECU multiplug, replug it onto the ECU, and switch the ignition to position 2 (engine off). Measure the resistance between pins 11 and 29 - should be up 800 to 5000 ohms. If higher, change sensor, or investigate wiring - has a pin pushed out of the new plug, or is there a dodgy joint?. (info above courtesy of Simon@SECS).