Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
ECM Code EO27 - Open Throttle Switch Circuit: Notes
TEST CONDITIONS: Always tested.
FAILURE CONDITIONS: Throttle switch was never closed during last key cycle. Throttle switch has not closed in current key cycle.
ACTION: ECM turns on "ENGINE CONTROL SYSTEM" light. ECM commands BCM to display "SERVICE SOON" message on CCDIC panel. ECM assumes throttle switch is closed if TPS position is less than or equal to 12 degrees, and brake switch is open (braking).
The throttle switch is part of ISC motor assembly. Pin "B" of ISC motor 4-pin connector is the throttle switch input to the ECM. The ECM provides a 4.5-6.0 voltage signal to pin "B" of ISC. When throttle lever contacts ISC plunger, throttle switch closes, shorting voltage signal at pin "B" to pin "A", which is a ground (closed throttle equals throttle switch input voltage low). Code EO27 is designed to detect a throttle switch that is always open.
- Code EO27 is due to throttle switch input open, ISC throttle switch inoperative, and should result in ECM input code EI72 not cycling. ECM input code EI72 will be high at all times.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.