Laars lx won't ignite

I'm not sure where the flame sensor is on that one, but the problem is the flame sensor is dirty. Also called the flame rod, flame proving rod, flame proving sensor. Should be a small rod that the flame from the main burner touches. Clean it with some sandpaper or replace it

I've got an easy way for you to check, and then we can help you from there.
First pop open both lids to your furnace (this may require a screwdriver). Turn up your thermostat, push the safety interlock button in if it has one (it's a button that turns off the power when the doors are removed), and watch.

On most modern furnaces (past 20-25 years) you'll hear/see:
1. Inducer fan
2. orange glow (ignitor)
3. Gas valve opening and ignition (flames)
4. Main Fan.

that's normal heater operation.
The most common issue here is a burned out ignitor, in which case you wouldn't see the orange glow but the sequence would attempt to continue anyway (the gas valve will open and hiss gas, but it won't lite, then it'll give up and try again a few minutes later).

On older furnaces there's a pilot light to start with, the click of the gas valve and boom... ignition.

On new furnaces it's
Inducer fan
gas valve opening
spark - ignition
Main fan.

and there are some oddballs in between, particularly the Lennox Pulse. If that's your furnace just back away slowly and call a professional... some guys won't even touch those, so be sure to mention what it is... if you call a pro.

So go ahead and watch it run, then tell me/us what you see.

Several steps here to check...
turn power off unit and back on
set thermostat to heat
watch for pilot to ignite..igniter should glow orange or cherry red or spark if its a spark type ignitor
Did you see a pilot light ignite?

If not you probably have a bad igniter.

If pilot lit....in a little bit you hear a click of the gas valve opening..then the main burners will ignite

If you see the pilot and the main burner lights but goes out in a few minutes or immediately you may have an issue of the "high limit" switch tripping.

Is the blower fan turning that blows air out of your vents?
are all your vents open in your house? too many times i see homes with almost all vents shut.

This error is Automatic Gas Shutdown. Here is what you can do:

1. Check for fluctuating pump pressure (air in strainer, pressure guage bounce)
2. Check all wiring connections
3. Replace ignitor/flame sensor assembly

Depending on your expertise, it could also be the gas valve or ignition control (aka the IID).

Since the burners do fire briefly, I would suspect the flame sense rod first. Attached is a link to the owners manual in PDF.

Lo Nox heaters can be a little tough so follow the directions exactly.

could be blocked orfice , mix some water and washing up liqued in bowl make bubbles and brush this on the pilot joints bubbles up you have aleak

One possible reason could be a bad thermocouple. This is a rod shaped device that is close to the flame. It's function is to turn off the gas if the pilot light goes out. Heat from the flame operates the thermocouple. When the flame goes out the thermocouple cools off and shuts off the gas. When one goes bad it will not let you light the pilot. Here's a picture of what one looks like. There's a bracket that holds it in place.

This is probally your ignition wire is bad and not reading flame at the pilot or your pilot is bent or something so that when the pilot lights the flame isn't hitting the sensor probe at the pilot. When looking under the heater use a mirror so if you get roll out of the flame you don't get burned. Good luck and be careful.

Clean the pilot tube of rust or other debris.

usually indicates a flame sensing problem. remove sensor and clean with steel wool .