Powertrain, generic · Auxiliary emission controls

P04D4 Exhaust Brake Input Circuit Range/Performance

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. P04D4 is the plausibility fault: the signal is electrically valid but does not fit the operating state – for example a button reporting pressed for an unusually long time, or a lever coding between two stages.

Likely causes

Button mechanically stuck or contact dirty (permanently pressed)
Contact resistance in connector or ground (voltage between thresholds)
Wrongly retrofitted or coded button or lever
Engine module or software

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

P04D3P04D5P04D6P04D7P0476

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