Engine starts, runs 1 sec. I'm told it's the alarm system and I need to replace computer

Dis arm the alarm with your key fob.

Not sure about yours, but many will reset by: turn key to "on" and wait 15 minutes or so; shut off, then try to start.

Ok. You don't tell us what year, make, or model the truck is, but I'm assuming it is a later model. They all come with integrated alarm and immobilizer systems installed. If you did not get an aftermarket adapter to disable the factory alarm system, then if the vehicle is started without the key it will shut off and alarm after 30 seconds. Once you used the key, you satisfied the manufacturers alarm system and then your remote start worked. You need to get a module specifically designed for your vehicle and alarm system to use with the Viper to make it work. They can be a real pain to get working correctly. Good luck!

Your Sonata is equipped with a "Keyless Entry & Burglar Alarm" system. When the Burglar Alarm is activated, it KILLS the starting system through the used of a relay called the "Burglar Alarm Relay". This mode prevents engine starting with the key.
Solution #1
Disarm the system with the Keyless Remote (key fob) If you don't have this remote (or its battery is dead) replace the battery & try disarming again.
Solution #2
Enter the vehicle & insert the key in the ignition and turn the ignition switch to 'RUN'. Wait 3 to 5 minutes until the alarm stops. You should then be able to start the engine.

Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

most of the alarm systems reset procedures are similar,lock with key/hold over for 30 secs then unlock/hold over 30 secs.(or vice versa)
OR press/hold unlock button on remote 30 secs,then press/hold lock button
OR put key in ignition,turn onto running position(not starter)and press/hold alarm button 30 secs
once car starts, it will remember codes in keys.

Check all of the vacuum connections. The brake booster vacuum line.

problem one is the timer control for the heaters ,also check the heaters that they are working ,oil problem ? look under the rubber boot that goes over the oil switch ,pull it back and if oil under the cover renew the oil switch,these motors do suffer from heater light flashing on and i know people have spent megabucks trying to cure it ,its a fault with dash and printed circuits ,but the heaters are not coming on i know because i have driven one with the heater circuit light coming on and i had a extended light on a long wire and the heaters are not coming on ,ignore it just renew the faulty glow plugs,as for EGR valve just pull teh vacumn pipe off the ERG valve and leave disconnect ,more hassle than its worth changing it ,you could end up with a ECU computer .Still a nice reliable motor though and economical

You will be best to connect the real tach wire. You will need to programme the alarm to recognise the speed of the engine, this way it will know where the idle position is.

Have you tried to switch your car on, wait 3 sec's and then start the engine? reason: If the pressure in the fuel system went down very fast, it need some sec's to arrive the pressure it needs to start. Also possible: switch on, wait 2 sec's, switch off, on and start. This does the same I told above. If that works, change the fuel pump or let this item checked be a specialist (it's expensiv to replace it).

Try this:
1.Get in the car
2. Closw the door
3. Disable the alarm sensor
4. Activate the alarm
5. Wait 30-35 sec
6. Get out of the car
7. Stop the alaram
8. Get in the car
9. Put the key on the position II, without starting the engine
10. Push the button READ

The alarm led must blink. Then you alarm is reset.
Sorry but my english is not very good, i'm from Romania, i hope you understand