What sensor do you replace on a 1999 mercury sable that stalls after 20 min of driving then starts and runs fine 45 sitting no engine light just smogged is not hot?

The intermittent stalling after 20 minutes of driving on your 1999 Mercury Sable, followed by a period where it runs fine after sitting, without any check engine light, points towards a sensor that's affected by heat. The fact it's not overheating suggests it's not a coolant temperature sensor directly.

The most likely culprit in this scenario is a Crankshaft Position Sensor (CKP) or, less likely, a Cam Position Sensor (CMP).

Here's why:

* Heat Sensitivity: These sensors can become unreliable when they get hot. After 20 minutes of driving, they reach operating temperature, and a failing sensor might begin to malfunction. The cooling-off period allows it to function again until it heats up again.

* No Check Engine Light: Sometimes intermittent failures of these sensors don't trigger a check engine light, especially if the problem is only present under specific heat conditions.

* Stalling: A faulty CKP or CMP prevents the engine from receiving the proper timing signals, leading to stalling.

Before replacement:

* Check connections: Ensure the CKP and CMP sensor connectors are clean and securely connected. Corrosion can cause intermittent problems.

* Visual inspection: Inspect the sensors themselves for any obvious damage or debris.

Important Note: While the CKP/CMP sensors are the most likely candidates, there's a small chance it could be something else. A proper diagnosis ideally requires a mechanic to use a scan tool to monitor sensor data in real-time while the car is driven. However, given the symptoms, replacing the CKP is a good starting point. If that doesn't solve it, then the CMP sensor is the next likely suspect.