P04A1 - Exhaust Pressure Control Valve 'B' Low

Repair Importance Level: 3/3
Repair Difficulty Level: 3/3   
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Possible causes

  • Faulty Exhaust Back Pressure Valve
  • Exhaust Back Pressure Valve harness is open or shorted
  • Exhaust Back Pressure Valve circuit poor electrical connection

Possible symptoms

  • Engine Light ON (or Service Engine Soon Warning Light)

P04a1 Description

Exhaust Back Pressure Regulator - Is a variable position valve that controls exhaust back pressure during cold ambient temperatures to increase cab heat and decrease the amount of time needed to defrost the windshield. The Engine Control Module (ECM) uses the measured exhaust back pressure, (ambient) intake air temperature, engine oil temperature and engine load to determine the desired exhaust back pressure. Valve position is controlled by switching the output signal circuit to 12 volts inside the ECM. On/off time is modulated from 0 to 99 % dependent upon the exhaust back pressure desired.

Upon each initial engine start-up, the ECM exercises the exhaust back pressure valve by commanding it to cycle on and off once. If upon start-up the temperature of intake air sensed by the Intake Air Temperature (IAT) sensor is < 37°F (5°C) and the oil temperature sensed by the Engine Oil Temperature (EOT) sensor is between 32°-140°F (0-60°C) it will continue to keep the back pressure valve partially closed. As the oil temperature rises during engine operation, the ECM will control the valve in response to the rising oil temperature.

OBDII Code P04a1 - Exhaust Pressure Control Valve 'B' Low - AutoCodes.com

P04A1 Information for specific makes

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P04A1 Comments

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