Body, generic

B0420 Audio System Internal Failure

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

No public J2012 list carries B0420 with this meaning, and no carmaker describes an audio-system fault under it. The one documented use is unrelated: Saab files B0420 06 in the 9-3 (2003–2012) climate control as an open or grounded circuit to the right air-mix flap motor, which upsets temperature control. Read literally, the title points to a fault inside the head unit, the part that feeds every speaker channel. Saab's 2011 9-5 shows how such units judge their outputs: the radio stores B1025–B1055 for a shorted or open speaker line and mutes only that channel. Klippel describes how the DSP, amplifier and microphones already fitted in a car could run a self-test of the whole system. Lost bus contact with the radio belongs under U0184.

Likely causes

Internal electronics or software of the head unit faulty
Low or unstable supply voltage or poor ground at the head unit
Unit overheated because of blocked ventilation behind the dash

Symptoms

  • No sound from any speaker, or the radio restarts by itself
  • Head unit freezes or ignores its buttons
  • Code stored together with other infotainment faults

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–2,000
Time0.5–3 h

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

Parts involved

audio head unitinfotainment module

Related codes

B0425B0455U0184

References: saabwisonline.com, saabwisonline.com, klippel.de.