CEL Codes understanding
 

I have a 1995 E320 which shows code 5 and 26 (EGR and upshift delay) before changing any parts I wanted to see if a leaky head gasket would cause a false code shut as these. Also I'd like a better understanding what 26 is telling me. Thank for the help Dave

Code 26 happens when the very 1st 2-3 shift of the day isn't at the correct engine RPM.
If the engine doesn't reach a specified RPM reading 2 days in a row on that shift the DM will flag that code.

Vacuum leak at the control pressure diaphram is the most likely problem.