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U0599 Invalid Data Received From DC/DC Converter Control Module A

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

On start-stop vehicles a DC/DC converter can hold the 12 V supply for sensitive electronics while the starter pulls the voltage down; on hybrids it feeds the 12 V network from the high-voltage battery. GM's 2025 engine tables check the converter's output voltage and crank-control messages for alive counter, protection value and checksum errors and set U0599, Type B, after 8 failures in 18 samples with at least 11 V for 5 s. GM bulletin 20-NA-191 for the 2020 Cadillac CT4 and CT5 with auto start/stop shows the code set falsely: the ECM miscalculated the protection value during cranking without boost, disabled start-stop on the first failure and lit the MIL on the second; an ECM reprogram fixed it. FCA lists U0599 for the 2019 Cherokee V6 ESS; Ford's 2014 list has it without MIL.

Likely causes

Engine controller software miscalculating the converter's protection value (GM 20-NA-191)
DC/DC converter A electronics faulty
Weak 12 V battery causing deep voltage dips during cranking

Symptoms

  • Auto start/stop disabled
  • Check engine light after repeated failures

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–900
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

dc dc converter module a

Related codes

U0298U059AP0A94

References: static.nhtsa.gov, gsi.ext.gm.com, static.nhtsa.gov, performanceparts.ford.com.