Ecu error code 121. p code 1693

Yes the computer will reset itself if the trouble goes away. This is common thing. You probably had something in your traction slip. I would not worry about it.

Auto Zone does not test the alternator under full load so the test result could be borderline.
start the car and turn everything on. If the voltage drops lower than 14.2
replace the alternator.
also the battery could be being drained overnight. to check use a test light by removing a battery terminal and connecting light between battery and cable with car off.
if it lights start be removing fuses.
when the light goes off the problem is with this circuit.
lastly if the light stays on take the wires off the alternator.
if the light goes out replace alternator.

It is best that you have a scanner used on your car to retrieve the error codes. The sound that you are hearing may or may not be related to the error the car's ECU has detected. Once you get the error code you can accurately diagnose the problem.

It would have to run for about 15 minutes for the codes to show up. That code is P1651 Fan1 Relay Control Circuit.Here is a code page for GMC.

http://www.engine-light-help.com/gmc-check-engine-light.html

You are describing low fuel pressure (fuel pump) problem. When you say you replaced the fuel filter, where did you find this filter on the 2004 Sonata? The problem, there is no Fuel Filter, per se, there is a 'SOCK' at the bottom of the fuel pump assembly which serves as a filter, inside the fuel tank. By the way, you mentioned the Check Engine Light came on. Did you scan for any codes in the computer (ECU)? If you did, what were the codes?

I am having the exact same problem with my 2001 Hyundai Elantra. I was told that the engine code that comes up when the sensor is on is called "Absolute Pressure In The Manifold". I was told by a "Hyundai Specialist" that when that code comes up it usually means that the seal on the manifold is Fuct. When my mechanic checked that out for me he had noticed that my manifold had 4 cracks in it(can varify cause he showed me). He replaced the manifold and gasket. The car ran fine for about 2-3 days and since then the problem has since come back and my check engine light will no longer go on/off it only stays on. Because of the check engine light always staying on I have noticed that on many occasions my car will hiccup many times in a row. The only way to stop the hiccup I have noticed is to take my foot right of the gas for approx. 2-3 sec then car will continue without hiccup(till it happends all over again). All in all I have been having this problem now for approx. 1 year and if there is somehow someway it could be repaired I would appretiate it.

CHECK ENGINE light??? Whenever the 'CHECK ENGINE' (Service Engine Soon) light comes on, the ECU (Engine Control Unit) sets a Diagnostic Trouble Code (DTC). These codes identify the particular engine management system which caused the light to come on.
You'll need to use/find an OBDII (On-Board Diagnostic) Scanner to retrieve any code which have been stored in the ECU computer. Afterward, we might be able to help you DIAGNOSE the problem and perhaps eventually fix the problem.

have the car checked when the check engine light is on,also a good scanner will be able to give you the history codes,get back to me with the codes and we'll go from there. you need someone with a professional scan tool

I'm having same problem on my 2000 model rodeo 4wd 3.2L V6. Other day check engine light came on. I checked engine bay nothing abnormal, or power loss. Next day light was gone. 2 days later driving, a sudden loss of power and light came up. After idling for a couple of minutes, drove off with light still on but all power back. 1 hour later of faultless driving loss of power for a minute or so. Engine temp normal. Had break for 1/2 hour and set off for last hour of journey without fault except check engine light still on. Following dayon startup light went out???
Seeking help from experts.

if your car is 2003+ try turning the key to run (but not crank) then to acc. 3 times and the engine codes will show up on the odometer, then you can look up what those codes are to help diagnose some of the problems.