Powertrain, generic (extended)

P2B5C Engine Coolant Flow Control Valve Supply Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

With P2B5C the supply voltage reaching the driver of the coolant flow control valve motor is too low. GM monitors this on the 3.0-litre Duramax (25OBDG04B): if the motor driver sees less than 6 V while the powertrain relay delivers more than 11 V, the difference points at the feed path, and a Type B fault follows after 80 hits in 100 samples. Typical causes are a blown or loose fuse, a corroded relay contact or a damaged supply wire; 21-NA-149 includes P2B5C among the LM2 codes linked to a chafed harness. Without enough voltage the motor cannot position the valve, so engine temperature control falls back to a default. A weak battery alone rarely sets it, because the relay check would fail as well.

Likely causes

Blown or loose fuse, corroded relay contact in the valve feed
Supply wire damaged or chafed in the engine harness
High resistance at a ground or splice of the feed circuit

Symptoms

  • MIL on after two drives
  • Valve does not move, temperature control in fallback mode

Repair estimate

DifficultyMediumDIY possible
Parts and labour€10–400
Time0.3–3 h

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

Parts involved

engine coolant flow control valvewiring harnessecm

Related codes

P2B58P2B59P2B5B

References: gsi.ext.gm.com, static.nhtsa.gov, law.cornell.edu.