My Buick Rendezvous now cranks but wont start no matter the temp of the engine or outside. Replaced the ECT sensor and it worked for a while but now refuses to start at all. Jumping it doesn't help.

Hi Julia , why do you believe you need to find the temp sensor ? Temp gauge not working ? The cooling Temp sensor is a input to the PCM / ECM - engine computer .
Engine Coolant Temperature Gage
The instrument panel cluster (IPC) displays the engine coolant temperature (ECT) as determined by the powertrain control module (PCM) or the powertrain control module (PCM). The IPC receives a class 2 message from the PCM indicating the ECT. The range on the ECT gage is between 60-125°C (140-260°F). The ECT gage defaults to 60°C (140°F) or below if the PCM detects a malfunction in the ECT sensor circuit. Refer to Cooling System Description and Operation .
Would have DTC- diagnostic trouble code stored in Instrument cluster or the check engine light would be on . Before replacing a part you may not need take it to a qualified repair shop to have it diagnosed .

Before you replaced parts did you feel the upper radiator hose ,as close to the engine as possible , From a cold start you would be able to tell if thermostat was opening ,by change in temp ! Check heater hose's going through the fire wall to inside vehicle . When vehicle is warmed up are both hose's hot to the touch ? I would suspects the temp . gauge before the coolant temp. sensor , Because I know GM has had problems with the stepper motors that control the gauges .
The instrument panel cluster (IPC) displays the engine coolant temperature (ECT) as determined by the engine control module (ECM) or the powertrain control module (PCM). The IPC receives a class 2 message from the ECM/PCM indicating the ECT. The range on the ECT gage is between 60-125?°C (140-260?°F). The ECT gage defaults to 60?°C (140?°F) or below if the ECM/PCM detects a malfunction in the ECT sensor circuit. Refer to Cooling System Description and Operation.
Class 2 is a serial data communication network ,found on GM vehicles , Vehicle computers share information on this network .
How's the antifreeze ? does the vehicle still have the original dex-cool antifreeze ? You haven't change the coolant with green coolant ? Dex-cool an regular green coolant don't mix , they gel up clogging heater cores ,passages in the engine block etc...

Won't crank or wont start?

Sounds like a bad temperature sensor to me. Temp sensors on newer cars tell the PCM to send more fuel on a cold start. If its malfunctioning then the PCM doesn't up the fuel. This fuel boost acts like the choke on an older vehicle

The only way to do it is to put it on a scanner and read the computer. But a good sign it isn't working is the temp gauge doesn't move. The ECT sends the signal to the computer(ECM), then the computer sends a signal to the gauge. Check engine light on?

This is usually a sign that the crank shaft sensor is bad.

Hi usually the symptoms your having are a result of the oxygen sensor(or lambda sensor for its technical name) being at fault.

now the reason it points to this being at fault is that it only starts working when the engine is near normal temp then if the car wont start till its cooled down then this is because the sensor then stops working when its cooled down so your car will run again until it gets hot,

you need to check where the sensor is located in the exhaust if it comes before the catalytic converter then the chances are its the sensor at fault but if it comes after the cat then it could be the cat that is at fault.

my best guess would be the sensor as your car is playing up exactly as if this sensor was not working.

i hope this is some help to you

let me know if you need further assistance ok

plz rate this solution as i have a whole page of unrated posts, thanks

Sounds like a coolant temp sensor. Have it scanned, Autozone will do it for free. If the computer has a code in it for the coolant temp sensor it will lock the cooling fans on to prevent overheat because it has no idea what the engine temp is.

Replaced outside temp sensor