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U04A4 Invalid Data Received From Particulate Matter Sensor A

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

Diesel particulate matter (PM) sensors have their own control unit that measures soot on the element and sends the result to the engine controller. GM's 2025 ECM tables (25OBDG04A) check the alive rolling count, protection value and checksum of about twenty soot sensor signals and set U04A4 after 3 bad frames in 10, with battery voltage of at least 11 V for 5 s; it is a Type B code, so the MIL comes on after two trips. VW's 2014 Touareg TDI chart sets it when the sensitivity factor leaves 0.75 to 1.25, when an invalid sensitivity signal lasts 2 s or when sensor messages fail for 0.6 s. Ford's 2014 chart also lists it as a MIL code. U02A3 is total loss, U04B2 the second sensor, U05A4 the reverse direction. The filter monitor pauses until data are valid.

Likely causes

PM sensor control unit or sensing element faulty (heat, soot, moisture)
Replacement sensor of the wrong variant or not yet matched
Damaged wiring or connector between sensor unit and engine controller
Low supply voltage disturbing the sensor unit

Symptoms

  • Check engine light on
  • Particulate filter monitor not completing, readiness missing
  • Usually no change in how the engine runs

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–700
Time0.5–2 h

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

Parts involved

pm sensor a

Related codes

U02A3U04B2U05A4P24AF

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