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Technical Description of OBD-II Trouble Code
High Circuit Temperature Sensor for Catalyst (Bank 2, Sensor 1)
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to OBD-II equipped vehicles with a catalyst temperature sensor (Subaru, Ford, Chevy, Jeep, Nissan, Mercedes-Benz, Toyota, Dodge, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
The catalytic converter is a crucial component of a vehicle’s emissions equipment. Exhaust gases pass through the catalytic converter, where a chemical reaction occurs. This reaction converts carbon monoxide (CO), hydrocarbon (HO), and nitrogen oxides (NOx) into harmless water (H2O) and carbon dioxide (CO2).
Converter efficiency is monitored by two oxygen sensors: one mounted upstream from the converter and one mounted downstream. By comparing the signals from the oxygen sensors (O2), the powertrain control module (PCM) can determine if the catalytic converter is functioning properly. A standard zirconium prealyst O2 sensor rapidly switches its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volts indicates a lean air/fuel mixture, while 0.9 volts indicates a rich mixture. If the converter is operating correctly, the downstream sensor should consistently read around 0.45 volts.
Catalytic converter efficiency and temperature are closely related. If the converter is functioning properly, the outlet temperature should be slightly higher than the inlet temperature. The old rule of thumb was a difference of 100 degrees Fahrenheit, but many modern vehicles may not exhibit such a significant difference.
There is no actual “catalyst temperature sensor.” The codes mentioned in this article are referring to the oxygen sensor. The “bank 2” portion of the code indicates that the issue is with the second bank of the engine, which does not include cylinder #1. “Sensor 1” refers to the sensor mounted upstream of the catalytic converter.
Trouble code P0438 is set when the PCM detects a signal from the catalyst temperature sensor, usually indicating an open circuit.
Related diagnostic codes include:
Code Severity & Symptoms
This code has a moderate severity. Symptoms of a P0438 engine code may include:
Causes
Possible causes for this P0438 code include:
Diagnostic and Repair Procedures
Begin by visually inspecting the upstream oxygen sensor and its corresponding wiring. Look for loose connections, damaged wiring, etc. Also, visually and audibly check for exhaust leaks. An exhaust leak can trigger a false oxygen sensor code. If any damage is found, repair it as necessary, clear the code, and see if it reappears.
Next, check for any technical service bulletins (TSBs) related to the issue. If none are found, proceed with step-by-step diagnosis of the system. The following is a general procedure, as testing for this code varies between vehicles. For accurate testing, refer to a diagnostic flow chart specific to the make and model of the vehicle.
Check for other DTCs
Oxygen sensor codes are often set due to engine performance problems that cause an imbalanced air/fuel mixture. If there are other DTCs stored, address those first before proceeding with oxygen sensor diagnosis.
Check Sensor Operation
This is best done using a scan tool or, ideally, an oscilloscope. Since most individuals do not have access to a scope, we will cover oxygen sensor diagnosis using a scan tool. Connect the scan tool to the OBD port under the dash. Turn on the scan tool and select the Bank 2 Sensor 1 voltage parameter from the data list. Bring the engine up to operating temperature and observe the sensor operation on the scan tool in graph mode. The sensor should rapidly switch between rich and lean (0.1 volts and 0.9 volts). If the sensor response is sluggish, it is likely faulty and should be replaced.
If the sensor constantly reads above 0.55 volts, either the sensor has failed, the air/fuel mixture is too rich, or there is an open in the sensor signal circuit. If the sensor constantly reads above 0.35 volts, either the sensor has failed, the air/fuel mixture is too lean, or there is high resistance or a short in the signal wire to the PCM.
Check the circuit
Oxygen sensors produce their own voltage signal, which is sent back to the PCM. Before proceeding, consult the factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles, and ALLDATADIY offers single vehicle subscriptions. To check for continuity between the sensor and PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (with the ignition off) between the O2 sensor signal terminal on the PCM and the signal wire. If the meter reads out of limits (OL), there is an open circuit between the PCM and sensor that needs to be located and repaired. If the meter reads a numeric value, there is continuity.
Next, check the ground side of the circuit. Turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (with the ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit that needs to be located and repaired. If the meter reads a numeric value, there is continuity to ground.
Finally, check that the PCM is processing the O2 sensor signal properly. Leave all the connectors attached and insert a back-probe meter lead into the signal terminal at the PCM. Set the digital multimeter to the DC volts setting. With the engine warmed up, compare the voltage reading on the meter to that on the scan tool. If the two do not match, the PCM is likely faulty or requires reprogramming.
Professional Service
While we always recommend seeking mechanical solutions for this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
