Navistar International DT 466 Diagnostic/Troubleshooting Manual page 511

7 ELECTRONIC CONTROL SYSTEMS DIAGNOSTICS 507
DTC 121 = Signal voltage was > 4.9 V for more than 0.4 second
DTC 122 = Signal voltage was < 0.039 V for more than 0.4 second
DTC 123 = Detected boost signal voltage was > 115 kPa (17 psi) absolute at low idle.
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.