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U0655 Lost Communication With Diesel Intake Air Flow B Position Sensor

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Some diesels carry a second intake air flow valve, labelled B; which flap gets that letter depends on the engine layout and the maker. When its angle sensor is a digital SENT unit, U0655 reports that the engine controller no longer receives usable position frames from it. No manufacturer documents this code in public. GM's 2025 diesel tables give SENT link codes only to valve A (U0654, Type A, 6.25 ms checks), and CARB requires a separate code whenever a digital input stops communicating. Analog sensor faults on the B side are P2396 to P239A, drive faults P2390 to P2395. Expect the flap to be switched off and left to its spring, with EGR control and regeneration limited and a likely MIL on emission-certified engines.

Likely causes

Contaminated or loose connector at throttle unit B
Signal, supply or ground wire to sensor B damaged
Digital position sensor of valve B failed
Engine controller input fault

Symptoms

  • Check engine light
  • Particulate filter regeneration postponed or blocked
  • Little change in drivability in many cases

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–700
Time0.5–2.5 h

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

Parts involved

diesel intake airflow position sensor bvehicle bus wiring

Related codes

U0654P2390P2391P2396P2397P2398P2399P239A

References: law.cornell.edu, gsi.ext.gm.com, en.wikipedia.org.