Powertrain, generic · Auxiliary emission controls

P04D6 Exhaust Brake Input Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

The exhaust brake input is the driver's request for the exhaust or engine brake – a button or stalk on diesel pick-ups and trucks. On GM's 6.6-litre L5P, for example, the button must be pressed again after every start. GM uses the exhaust brake status as an enable condition for other emission monitors, which is why CARB requires the input to be checked. No manufacturer table available to us lists P04D3–P04D7; Hino's J08E truck engine checks P04D5 before its exhaust brake function test. P04D6 means the input voltage stays above its upper threshold – the signal line is shorted to the supply, or with a pull-up input the button ground or signal line is open.

Likely causes

Signal line shorted to supply voltage
Button ground or signal line open (pull-up logic)
Button faulty internally
Corroded or wet connector

Symptoms

  • Exhaust brake cannot be switched on, button lamp off or wrong
  • Possibly check engine light
  • More load on the service brakes downhill

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–300
Time0.5–2 h

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

Parts involved

exhaust brake switchswitch wiringengine control module

Related codes

P04D3P04D5P04D4P04D7P0478

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