Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P05B9 Restraint Deployment Signal Circuit Low

Check engine lightUsually off

EmissionsNot relevant

Drive cycleNot required

Limp modeNo

After a crash the restraint control module tells the engine module that airbags or belt tensioners have fired; the engine module then cuts the fuel pump (Ford: until the ignition is cycled) and possibly the engine. Depending on the maker the message travels over CAN and/or a dedicated wire with a defined level or voltage drop. P05B9 means the signal line stays below its valid rest level – usually a short to ground. Where deployment is coded as a voltage drop, a short to ground could resemble a deployment, so makers cross-check with the CAN message. No manufacturer document available to us uses this code; it is a safety function, not an emission-relevant monitor, so usually without warning lamp.

Likely causes

Signal wire chafed to ground (harness under seat or console)

Water in the connector (footwell)

Restraint module output stuck low

Engine module input

Symptoms

Code stored
Possibly fuel cut-off warning
No check engine light

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€80–300

Time0.5–2 h

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

Parts involved

restraint control modulecrash signal wiringengine control module

Related codes

P05B8 P05BA P05BB P0230 U0151

References: static.nhtsa.gov, patents.google.com, law.cornell.edu.