What is an ECL? (//ww2.arb.ca.gov/engine-label-or-emission-control-label-ecl)
https://ww2.arb.ca.gov/engine-label-or-emission-control-label-ecl (//ww2.arb.ca.gov/engine-label-or-emission-control-label-ecl)

What to do if the ECL missing/illegible

All heavy-duty vehicles need to have proof that the engine meets appropriate emissions standards by having the manufacturer ECL properly affixed on the engine and fully legible.  If you are unable to obtain the ECL, you must request the manufacturer write you a letter stating the label is unavailable. Engine information must be provided on the manufacturer's letter.  A copy of the letter must be kept in the cab of the vehicle in a place known to the driver.

Heavy-duty vehicles manufactured before 1974 or were originally not equipped with an ECL but have evidence that the vehicle meets Environmental Protection Agency (EPA) and CA emissions standards may note “Not Applicable” in the form submission section for “Engine Family Name”.