Wouldn't to know the mower model number that you working on.
Assumng DSAI and assuming you have a solenoid shift starter. Yes if the steering diode is shorted if on an older engine that previous operated correctly or if a brand new engine harness, a steering diode installed backwards.
The blue diamond would the suspected bad one; although, both may be bad as they are both steering diodes. Otherwords the setup is to allow +12 v to flow to the coils and not to back flow to the starter or other engine or mower systems. IF I got things straight in my mind the red wire going to coils is +12v hot when the switch is on.
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