Check engine light just came on

My 2003 mercury all of a sudden started running very rough and has a sulfer smell and the check engine light came on. I bought a code reader but no codes are being displayed.

2003 Mercury Mountaineer 85000 miles

service engine light came on after driving in some snow. No cowboy driving. No other symptoms. Curious as to what it could be.

Truck problems please expert help?
My check engine light is on code said p1299 could someone explain this. Before the check engine light turned on the check gage light would turn on and off every 30minutes lasted about 2 days like this then out of nowhere the check engine turn on and the check gage light was on also and even tho I would accelerate fast the rpm would go to 3 and speed to 10mph it wouldn't go any higher stayed like that for about 2 minutes and it was all shaky also just because I wanted to get home asap the truck didn't turn off but when I got home water was leaking on the bottom don't know exactly where but it was somewhere on the passenger center side. In the last 2 months its been nothing but waisting money I've changed the oil, changed the ac compressor because it was leaking antifreeze also changed the hoses connected to the ac compressor, changed the spark plugs because check engine light said so, and the radiator is about 2 years new

This code indicates that the engine cylinder head temperature sensor indicates the truck is overheating and going into a fail safe strategy. IBelow is a list and description of the trouble code. You need to first check the coolant level, if it is low then look for a external leak somewhere. You said that is seemed to leak where the thermostat is. In the center behind the alternator. Follow the upper radiator hose. I would pressure test the coolant system to find if you have more leaks than that one location.

Detailed Description
Indicates an engine overheat condition was detected by the cylinder head temperature (CHT) sensor. An FMEM Strategy called Fail-safe Cooling was activated to cool the engine.
Possible causes :-
Engine cooling system concerns.
Low engine coolant level.
Base engine concerns.

I turned my car on again and drove it couple blocks and it let's me drive it like their is no problem only thing that the check engine light is still on would it still be the sensor you mentioned. What happens if it falls on safety mode?

Fail Safe Cooling

The vehicle has a strategy built into the powertrain control module (PCM) that will control the engine if it starts to overheat.

Stage 1 of the strategy will commence if the engine starts to overheat. The cylinder head temperature (CHT) sensor transmits a signal to the PCM, which then moves the temperature gauge pointer into the red zone.

If the engine is not switched off and the temperature continues to rise, the Powertrain Check Lamp is illuminated. This indicates to the driver that the engine is approaching critical limits and should be stopped. At this point DTC P1285 is set in the PCM which can be retrieved using a scan tool.

Stage 2 of the strategy will commence if the lamp and temperature gauge are ignored by the driver. The PCM will start to control the engine by cutting out two cylinders and restricting the RPM to below 3,000 RPM. Simultaneously the malfunction indicator lamp (MIL) will be illuminated. This indicates that long term engine damage can occur and vehicle emissions will be affected. At this point DTC P1299 is set in the PCM which can be retrieved using a scan tool.

Air is then drawn into the deactivated cylinders. This helps to control the temperature of the engine internal components. The deactivated cylinders are alternated to allow even cooling of all the cylinders.

NOTE: If the driver is using a high percentage of throttle travel (for example, an overtaking maneuver) when the PCM starts engine deactivation (Stage 2), the deactivation will be delayed for 10 seconds.

NOTE: After 2-cylinder operation has begun, the engine will not revert to 4-cylinder operation, even if the temperature should fall, until the ignition is switched off and then on again.

NOTE: The malfunction indicator lamp can only be extinguished by using a diagnostic tool after the fault has been rectified and the DTC cleared.

Stage 3 of the strategy will commence if the engine temperature still continues to rise. This will result in the engine being totally disabled before major engine damage or seizure occurs. The Powertrain Check Lamp will begin to flash, indicating to the driver that the engine will be switched off after 30 seconds. This allows the driver time to choose a suitable parking place.

Driving the vehicle even more will cause more expenses. Rent a pressure tester for free at an auto parts place and find leaks to repair. You said it was where the thermostat location is so that might be a culprit as a thermostat stuck in the closed position will prevent coolant flow and cause the temperature to steadily rise. In a matter of minutes, if you fail to notice the problem, your engine will be destroyed

Oh ok thank you

My check engine light is on and at times I notice a slight "studder" when I am driving and I replaced the EGR valve and no change. The code that reads is as follows:
Exhaust Gas
Recirculation Flow
Insufficient
Detected

No I haven't. This was the hardest EGR valve to change :-) how do I check it?

My truck shakes when it is to a stop. While I drive no problem its all good but when I stop feels like if someone would be jumping inside. Went to auto zone said its the spark plug #5. How do I know if that's the only problem #5 looks like its leaking something is that another problem?

Okay I will do that Sunday after Christmas and see what it does. If that is it where would I go from there? If that isn't the problem what would be the next step? Thank you for your help!

Engine Performance problem
2002 Mercury Mountaineer V8 All Wheel Drive Automatic

My check light is on, I did my tune up and it went off untill 3days later. Engine still vibrates, my trucks use alot more gas and check engine light still on, what can it be