Useful Thoughts and Actions Taken

Error codes 16684 and 16685:  Missing on cylinder 1

• Google: vw error code 16684
• 16684 – Random/Multiple Cylinder Misfire Detected
• Google: vw error code 16685
• 16685 cyl 1 misfire po30135
• 16795 16685 16684 17658 Fault Codes – has a list of possible causes of misfire
• Youtube:Mk4 Golf Ignition Coil Replacement
• YouTube: MK4 Golf Spark Plugs and Spark Plug Wire Replacement  – They do not make it very easy. 
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16685: Cylinder 1 Misfire Detected
Possible Causes

* Ignition system
* Fuel injectors
* Fuel pressure
* Running out of fuel
* EVAP canister purge valve
* Evaporative emission system
* Low compression
* Base engine problems
* Controle Module damaged 

Special Notes: This feature search for engine misfire in a specific cylinder. Most of the time the cause for a misfire is a lack of combustion in a cylinder due to absence of spark, poor fuel metering, or poor compression.

16684: Random/Multiple Cylinder Misfire Detected

Possible Causes

* Air Intake System leaking
* Fuel Supply faulty
* Injector(s) faulty
* Ignition Cable(s) and/or Spark Plug(s) faulty
* Ignition Coil(s) faulty
* Exhaust Gas Recirculation (EGR) Valve (N18) stuck/open
* Camshaft Position Sensor (G40) faulty 

Possible Solutions

* Check Misfire Recognition
* Check Air Intake System
* Check Fuel Supply
* Check Injector(s) and Injector Sealing
* Check Ignition Cable(s) and Spark Plug(s)
* Check Ignition Coil(s)
* Check Exhaust Gas Recirculation (EGR) Valve (N18)
* Check Camshaft Position Sensor (G40) 

Many times after popping the misfire codes the car would also pop a P0420 Catalyst System,Bank1 Efficiency Below Threshold 16804.  It appears that the extra stray gases from incompletely burnt gasoline is more than the catalytic converter can handle. Most of the time this code is popped is after a misfire code but I have finally seen it with no misfire code.  However there seems to be a residual misfiring so perhaps the engine is misfiring below the threshold that pops the misfire code.  

Next debug step:  use a spray bottle to mist around the ignition coil and the plug wires to see if I can exacerbate the misfire problem.

Resolution:

• Spark plugs had a lot of wear.  I suspect they had never been changed on this car as of 92kmiles.  Replacement resulted in the misfire codes not returning.  See photo below.  This cleared most of the problem.