Wiring fault on the exhaust brake input circuit, such as a short to voltage or chafed harness; Corroded, damaged or loose connector at the exhaust brake component or control unit; Faulty exhaust brake switch, valve position feedback or actuator input device; Open circuit in the signal or ground wire causing the input to read high; Fault in the supply or reference voltage to the circuit; Internal fault in the control unit input stage.

P04D6

Exhaust Brake Input Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
D6: FaultThe specific fault in that area

What it means

P04D6 is a generic powertrain code meaning the control unit has detected a high voltage on the exhaust brake input circuit. The module saw a signal level above the expected range, which can point to a short to voltage, an open circuit that leaves the line pulled high, or a fault in the switch or sensor feeding that input. Exact circuit layout and what the input represents (switch, feedback or position signal) depend on the vehicle and engine.

Symptoms

Malfunction indicator lamp on, possibly with an exhaust brake warning light
Exhaust brake not working or working intermittently
Reduced engine braking on downhills
Exhaust brake operating when not requested, or not releasing
Other stored codes relating to the exhaust brake circuit or related supply

Possible causes

Wiring fault on the exhaust brake input circuit, such as a short to voltage or chafed harnesslikely
Corroded, damaged or loose connector at the exhaust brake component or control unitlikely
Faulty exhaust brake switch, valve position feedback or actuator input devicepossible
Open circuit in the signal or ground wire causing the input to read highpossible
Fault in the supply or reference voltage to the circuitpossible
Internal fault in the control unit input stagerare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including when the fault was set
Check for related codes covering the exhaust brake, supply voltage or reference voltage, as these may point to a shared cause
Inspect the exhaust brake wiring and connectors for chafing, heat damage, water ingress, corrosion and bent or pushed-back pins
Using the manufacturer wiring diagram, identify what the input is and its expected voltage with the exhaust brake on and off
Measure the signal voltage at the control unit and at the component with the connector attached and disconnected to separate a wiring fault from a component fault
Test the signal wire for a short to voltage and for continuity, and check the ground circuit for resistance
Check the switch or feedback device against specification and operate the exhaust brake to see if the reading changes as expected
Clear the code and road test to confirm whether it returns, only after the fault has been found and repaired

How to fix

Repair or replace damaged wiring and fix any short to voltage or open circuit
Clean, repair or replace faulty connectors and terminals
Replace the faulty exhaust brake switch, feedback device or actuator if it tests out of specification
Restore the correct supply, reference or ground connection if this is the cause
Replace or repair the control unit only after all wiring and components have been proven good, following the manufacturer procedure

Can I keep driving?

Medium severity

The car can usually be driven short term, but the exhaust brake may be unavailable or behave unpredictably, so allow extra braking distance and take care on long descents. Get it checked soon, especially if you tow or carry heavy loads.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.