Quad drivers are amplifier circuits. On the "C" engine #1 handles the SES light and canister purge solenoid. #2 the a/c and cooling fans relays, #3 the HOT light and TCC solenoid, and #4 the three EGR circuits. Factory Service manual has several pages of diagnosis. Note: code 26 sets because an electrical fault was detected (e.g. loose ERG connector). It will not set on a mechanical fault (e.g. stuck ERG).
If the SES light is acting properly, lockup is occurring, and the cooling fans are on with A/C then it is probably the EGR circuit.