LG Gas Dryer 5988W error is check filter

The check filter light is only a reminder. It is not connected to a sensor or anything. Supposed to stop flashing as load starts. If all else is working properly, I wouldn't worry.

check your door interlocks and connections

Check filter is just a reminder. Not connect to any sensor,etc. I would guess your igniter is bad. A common issue. Check out repairclinic.com or appliance parts pros.com for repair videos.

the check filter light is just a reminder. it is not connected to a sensor. goes out when you start the next cycle so your issue is not connected to that.

the blinking filter light is just a reminder, not connected to anything. goes out when you start cycle. however, if the dryer overheated then it may have blown the thermistor fuse. check out appliancepartspros.com or repairclinic.com for how to videos and parts.

the clean filter just blinks at start of a cycle to remind you to clean it. there is no lint filter sensor or anything like that. it goes out after you program your cycle and hit start. if dryer is not coming on, there is another issue. any fault/error codes?

Try cleaning out the compartment below the lint filter. There's a sensor in there that is probably buried in lint.

Think of this as a "low airflow" indicator. I know it says clean filter, but this error code is triggered when the temperature gets too high in the air duct sensor at the front of the machine as compared to the sensor at the rear of the machine. the most likely cause is a dirty filter. if you use dryer sheets, the fine mesh can get clogged with paraffin wax. if you hold your filter up to the light, and it looks cloudy and clogged, clean it in hot water to dissolve the paraffin.
if the light still comes on, check to make sure your ducting is not clogged and that the flapper door on the outside of your house where the duct exits is clear and that it opens when the dryer is running. you may need to unplug the dryer if the clean filter light is on and wont go off. plug it back in and try to run it.

My wife has a Bosch Premium dryer that had drying problems. The lint filter would get cleaned but the machine still did not operate correctly, as you explained above. Her machine was made in 2005 and the lint screen, the plastic slotted unit in front of the lint filter was the problem. It gets clogged up with hair/lint that never makes it to the filter. I tried a shop vac to vacuum it but the problem was still there. Get a pistol cleaning kit for a 22 cal gun. Pull the lint filter out & then take the pistol rod and insert it from the drum side into each slot and pull out the tons of lint. Pull the lint off the rod when done and take a flame to burn the rest off as the rod is a brass brush with small wires on it.

This can obviously be several things, but I would first check the small safety thermostat. On the gas models, there is a small pin sticking out of it which can be pushed back in to reset it. It is located on the heater housing and is only about the size of a dime, with two wires on it. This will trip out easily if your vent was not clean, or is long or restricted in some ways (like using flex ducting)
If you need more help getting to it, let me know and I'll give you more instructions.