Description of Flash Code 18
Flash Code 18 indicates that the Throttle Valve Plate Input Voltage has dropped below the 5% sensor supply voltage (normally < 0.25 volts). Typically, the problem is an open signal or an open sensor supply; refer to "18.3 Troubleshooting Flash Code 18, Throttle Valve Plate Input Voltage"
. This code also indicates that the voltage seen at the ECM for the relative humidity sensor signal wire was below programmed limits; refer to "18.4 Troubleshooting Flash Code 18, Relative Humidity Sensor Circuit Failed Low"