What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (BMW, Honda, Mitsubishi, Toyota, Ford, Jeep, Hyundai, etc.). Although generic, the specific repair steps may vary by make/model.
If you are faced with diagnosing a stored P0439 code, you should know that it means the Powertrain Control Module (PCM) has detected a problem with the heated catalyst control circuit for engine bank two. Bank 2 specifies that the malfunction occurred in the catalytic converter for the engine bank that does not contain cylinder number one. Conditions that can lead to this code being stored can be electrical or mechanical.
The main function of the heated catalytic converter is to reduce exhaust emissions from diesel and gasoline engines. It is a dense ceramic fiber and precious metal filtration element enclosed in a steel housing and positioned in the exhaust pipe. Harmful nitrogen oxides (N2O), carbon monoxide, and unburned hydrocarbons are all converted into harmless nitrogen, oxygen, carbon dioxide, and water ions after passing through the heated catalytic converter. This is accomplished using the filtration element and the extreme heat from the engine exhaust.
In the case of the heated catalytic converter, the filtration element is further heated with a PCM-controlled injection of a certain type of flammable diesel exhaust fluid. Catalytic converter temperatures must reach at least 800 degrees Fahrenheit, and heated catalyst units can reach 1,200 degrees Fahrenheit.
The efficiency of the catalytic converter is monitored using upstream and downstream oxygen (O2) sensors and exhaust temperature sensors.
If the heated catalyst is not functioning correctly, the variations in oxygen concentration between the input exhaust and the output exhaust will not be significant. If the upstream and downstream O2 sensors reflect a similar exhaust oxygen concentration, or if an electrical malfunction is detected in the heated catalyst control circuit, a P0439 code will be stored and a malfunction indicator lamp may illuminate.
Other catalyst efficiency trouble codes for bank 2 include:
P0430 Catalyst System Efficiency Below Threshold (Bank 2)
P0431 Warm Up Catalyst Efficiency Below Threshold (Bank 2)
P0432 Main Catalyst Efficiency Below Threshold (Bank 2)
P0433 Heated Catalyst Efficiency Below Threshold (Bank 2)
P0434 Heated Catalyst Temperature Below Threshold (Bank 2)
P0435, P0436, P0437, P0438 Catalyst Temperature Sensor Circuit Codes
Code Severity and Symptoms
Since the catalyst heater is crucial for reducing exhaust emissions, a P0439 code should be considered severe.
Symptoms of this code may include:
Decreased fuel efficiency
A lack of overall engine performance
Excessive black smoke from the exhaust
Other associated diagnostic trouble codes
MIL (Malfunction Indicator Lamp) illumination
Causes
Possible causes for this code being set are as follows:
Faulty exhaust temperature sensor
Incorrect or insufficient diesel exhaust fluid
Faulty diesel exhaust fluid injection system
Burned, chafed, broken, or disconnected wiring and/or connectors
Faulty O2 sensor(s)
Faulty catalytic converter
Engine exhaust leak
Diagnostic and Repair Procedures
For me, a diagnostic scanner, a digital volt ohmmeter (DVOM), an infrared thermometer (with a laser pointer), and a reliable source of vehicle information (like All Data DIY) will be necessary when diagnosing a P0439 code.
All diesel exhaust fluid injection codes, misfire codes, throttle position sensor codes, manifold air pressure codes, and mass air flow sensor codes should be addressed before attempting to diagnose a stored P0439 code. The engine must be in good working order before diagnosing this code.
I usually start my diagnosis with a visual inspection of the system’s wiring harnesses and connectors. I would focus my attention on harnesses routed near hot exhaust pipes and manifolds, as well as those routed near sharp edges like those found on exhaust shields.
Next, I would connect the scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes and freeze frame data. I write this information down as it can be helpful if this turns out to be an intermittent code. Clear the codes and test drive the vehicle to see if the P0439 resets.
If applicable, ensure the diesel exhaust fluid tank is filled with the correct fluid and that the injection system is operating correctly. If the diesel exhaust fluid injection system fails, the heated catalyst will not function efficiently and a P0439 code will be stored. If the diesel exhaust fluid injection system is not operating correctly, check the system fuses and relays to ensure the controller is functioning.
If the diesel exhaust fluid injection system is operating as expected, use the infrared thermometer to test the inlet and outlet temperatures of the catalytic converter. Use the vehicle information source and compare the actual temperature data with the manufacturer’s specifications. If the outlet temperature does not meet specifications, suspect a faulty catalytic converter.
If the heated catalyst outlet temperature meets specifications, use the DVOM to test the exhaust temperature sensor using the specifications found in the vehicle information source. Replace the exhaust temperature sensor if it does not meet the manufacturer’s specifications.
Test the O2 sensors following the manufacturer’s recommendations.
Additional diagnostic notes:
P0439 code is frequently stored due to incorrect or insufficient diesel exhaust fluid
Disconnect associated controllers before probing circuits with the DVOM