Hello!
The codes you have are defined as:
26 Quad driver fault
27 Transmission third gear switch fault
A quad driver is a switching transistor located inside the powertrain control module that is used to turn accessory devices on or off. Quad drivers on Gm vehicles are typically used ror such things as cooling fans, A/C compressor clutch, canister purge solenoid control, etc... You may want to take a look to see if anything electical is disconnected under your hood, or if anything electrical is obviously not operating.
The third gear switch in teh transmisison is used to tell the computer that the transmission is in third gear. If the switch output is lost for 10 seconds, this code stores. Since you do not mention any transmission issues, the problem may ahve been a temporary glitch; it could also be a failing switch or a paoblem with the electrical circuit between teh switch (inside teh transmission valve body) and the PCM.
What I would suggest doing fiorst is to disconnect the battery to clear the codes, and tehn driving the vehicole to see if the light comes back on and which code(s) are stored again. If either of them return, let me know and I will see if I can dig up the manufacturer's diagnostic chart for that particular code for you.
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Auto Service Technician
25+ yrs experience as a professional working technician; ASE L1 master technician