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Error Code P048B is defined as Exhaust Pressure Control Valve Circuit.
This error code is a generic trouble code, meaning it applies to all vehicles that come the with OBD-II system, especially vehicles made since 1996 up to present, and vehicles with exhaust pressure control valve sensor or switch. This includes vehicles from Audi, Toyota, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
The Exhaust Pressure Control or the EPC is a solenoid controlled valve used to regulate back pressure during cold temperatures. This is particularly helpful during the cold season, as it what aids the vehicle in heating the cabin, defrosting the windshield, and helps the vehicle during cold start. It is typically found in diesel engines.
In most cases, the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) uses the information from the EBP (exhaust back pressure) sensor, IAT (intake air temperature), and MAP (manifold absolute pressure) sensor to determine the control of the valve. If the PCM discovers the problem with the EPC or the IAT, it will automatically disable the ECP.
Error Code P048B is set when the PCM detects a malfunction in exhaust pressure control valve circuit, or if it’s out of factory specifications.
As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other symptoms include:
- Poor engine performance
- Hard starting
- Increase emissions
Common causes of this code are:
- Defective Exhaust Pressure Control Valve
- Wiring Problem
- Defective PCM
How to Check
Start the diagnosis for this code by checking the EPC valve and all connected wiring. Check for loose connections, damaged wires, etc. Repair any damages as necessary.
Check the TSB (Technical Service Bulletin) for the vehicle and its problem. Often, manufacturers provide in-depth information for an easy fix of the problem. If you don’t find anything, then proceed with the diagnosis. The following is a general procedure for this code. To get an accurate diagnosis for the system, refer to the manufacturer’s diagnostic flow chart.
Check the Wiring
Check the factory wiring diagram of the vehicle and determine which wires are which.
Test the Solenoid
Remove the solenoid connector and then check the solenoid for internal resistance using the digital multimeter. To do this, connect the meter between the solenoid B+ terminal and the solenoid ground terminal. Compare the resistance measurement to specifications set in the factory repair. If the meter shows reading that is out of specifications, or out of limits (OL), that indicates an open circuit, which means you need to replace the solenoid.
Check the Circuit’s Power Slide
When the vehicle is still cold (make sure it has sat for a few hours or overnight), remove the solenoid connector. With the vehicle ignition on, use a digital multimeter set to DC volts to check for power at the solenoid (usually 12 volts). To do this, connect the negative meter lead to ground, and the positive meter lead to the solenoid B+ terminal on the harness side of the connector.
If there is no voltage present, connect the meter set to ohms (still with ignition) between the B+ terminal on the solenoid connector and the solenoid supply voltage terminal of the PCM. If you get OL (out of limits) in the meter, that means there’s an open circuit between the PCM and the sensor, which must be located and repaired. If you get a numeric value, that means there’s a continuity.
If everything seems to be in good shape at this point, then you have to check power coming out of the PCM. Turn the ignition and then set the meter to DC volts. Next, connect the positive end of the meter to the EPC supply voltage terminal on the PCM, connect the negative lead to ground. If you don’t get reference voltage from the PCM, that means the PCM could be defective. However, it is important to note that PCM rarely go bad. Thus, make sure you double check at this point.
Check the Ground Portion of the Circuit
With vehicle ignition off, set the multimeter at ohms and then check for continuity to ground. Remove the solenoid connector. Connect the meter between the solenoid ground terminal and the chassis ground. If the meter reads a numeric value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and solenoid that will need to be located and repaired.
How to Fix
Depending on the result of your diagnosis, common repairs are:
- Repair or replace damaged connectors or wires in the exhaust pressure control valve
- Replacement of solenoid
- Locate and repair open circuit between the PCM and the sensor
- Repair or replacement of PCM
Again, the PCM rarely goes bad, but if you’re one of the unlucky ones, it is worth to consider. Just make sure you double check to pinpoint the fault.