Diagnostic LED decoding
 

What does diagnostic LED#4 translate to when solid On. This is showing on my 89 300SE with California emissions. There is 2 pushbuttons on the diagnostic modual, a short button under the LED modual cover and a larger one on the left side of the connector.
I tried to rest LED #4 using the tall reset button or both botton at the seame time without success, holding the small reset button for about 5 secs seems to reset the led but only for a short time before it comes back solid red. I don't notice any performance defernce while the diag led#4 is on.
Any idea please?