P04D6 code meaning
Exhaust Brake Input Circuit High
Powertrain system Generic code P04xx codes Check engine light
What P04D6 means
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.
P04D6 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of P04D6
- Exhaust brake cannot be switched on, button lamp off or wrong
- Possibly check engine light
- More load on the service brakes downhill
Common causes of P04D6
- Most likelySignal line shorted to supply voltage
- PossibleButton ground or signal line open (pull-up logic)
- PossibleButton faulty internally
- Less likelyCorroded or wet connector
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Exhaust brake switchSwitch wiringEngine control module
What each part of P04D6 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| D6 | The exact fault in that area. |
What to do next
- Note P04D6 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P04D6
What does P04D6 mean?
P04D6 means "Exhaust Brake Input Circuit High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P04D6?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P04D6 fail an emissions test?
Yes, it can. P04D6 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P04D6 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P04D6 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.