Error Code P0548 is defined as Exhaust Gas Temperature (EGT) Sensor Circuit Low Bank 2 Sensor 1. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, however, differ from one make and model to another.
Error Code P0548 means there’s a problem with EGT (Exhaust Gas Temperature) sensor, indicating a too “low” voltage condition. The EGT sensor is a crucial component located in the “up” pipe forward of the catalytic converter. Its purpose is to protect the converter from damage caused by excessive heat. By converting temperature to voltage and sending it to the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes). If the PCM determines that the voltage input being sent is lower than the predetermined limits set by the vehicle manufacturer, the code P0548 is stored.
It’s similar to the Error Code P0549, with the latter indicating too “high” voltage condition. Both codes have the same diagnosis and repairs, and they both refer to the Bank #2 (side of the engine that does NOT contain cylinder #1). Also, this code is the same with Error Code P0545, with the latter refers to Bank 1.
The EGT sensor is a heat sensitive resistor that converts temperature of the exhaust to a voltage signal for the computer. It receives a 5V signal from the vehicle’s computer through a wire, while the other wire is for ground. This sensor is found in most late gas or diesel model engines.
The higher the exhaust temperature is, the less resistance to ground it will have, resulting in a higher voltage. Conversely, lower temperature means more resistance, resulting in a lower voltage. If the engine sees a low voltage, the PCM will automatically work to vary the timing of the engine, or its fuel ratio, ensuring an acceptable temperature range within the converter.
For diesel engines, the EGT works by determining the timing of the regeneration of the PDF (diesel particulate filter based on the rise of temperature).
If a catless up pipe is installed within the removal of the catalytic converter, there will be no provision for the EGT, or it will simply not function correctly without the back pressure, which also results to this code.
Other related codes include:
- Error Code P0547
- Error Code P0549
As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. There are no noticeable symptoms, drivability, or performance problems that go with this code.
Common causes of this code include:
- Defective EGT sensor
- Broken, burned, corroded, or loose connectors, terminals or wirings
- Internal short in a sensor or to its ground
- Pressure issues (common among aftermarket exhaust systems when taken to off-road systems)
- Upstream coming off the sensor, or extreme leak in the exhaust system
How to Check
First thing mechanics do to diagnose this code is raise the vehicle to locate the sensor. This code refers to Bank 2 sensor, which is located on the engine side that does NOT have the cylinder #1. It’s found between the exhaust manifold and the converter. For diesel engines, it’s found before the Diesel Particulate Filter (DPF). It’s a bit different from the O2 (oxygen) sensor which is just a plug with two wires. For vehicles with turbo, the sensor is placed next to the turbo exhaust input.
Next, they will check the connectors and look for signs of damage such as corrosion or loose terminals. They will also check the pigtail up to its connector and check it as well.
Then, they will proceed by looking for signs of missing insulation or bare wires that may be short to ground.
Next, they will disconnect the upward connector and remove the EGT sensor. Then, check it for resistance. They will also probe both terminals in the connector. A good EGT will have around 150 ohms. If there is low resistance, say 50 ohms, then that means the sensor is faulty and must be replaced.
Next, they will use a hair dryer or heat gun and heat sensor to check the ohmmeter. The resistance must drop as the sensor is heated and rise as it cools. If it doesn’t, then it must be replaced.
If everything is good up to this point, turn the key on and measure the voltage at the engine side of the pigtail. The connector must have 5V. If it doesn’t, it’s faulty and must be replaced.
If these repairs failed to detect the problem, technicians will have to check the voltage at the sensor’s connector with the vehicle ignition turned at “on” position. If it has sufficient voltage, then the problem is in the PCM.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of defective EGT sensor
- Repair or replacement of burned, corroded, or loose connectors, terminals, or wirings
Another common cause for this code is if the catalytic converter has been replaced in favor of a catback system. This leads to uncontrolled emissions into the atmosphere, which is illegal for most states, and may result into large fines if caught.