P0425 – Catalytic converter temperature sensor, bank 1

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By Alexandros Paleologos (Contact Me)
Last Updated 2016-07-10
Automotive engineering student
CodeFault LocationProbable Cause
P0425 Catalytic converter temperature sensor, bank 1
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Wiring, poor connection, catalytic converter temperature sensor, ECM

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Table of Contents

  1. What Does Code P0425 Mean?
  2. What are the symptoms of code P0425 ?
  3. How do you troubleshoot code P0425 ?
  4. Codes Related to P0425
  5. Get Help with P0425

What Does Code P0425 Mean?

Nowadays vehicle with emissions control systems use heated oxygen sensors. The purpose of the heating element build into the oxygen sensor, is to get the sensor up to operating temperature (600 degrees) quicker and start the exhaust gases monitor before .The Engine Control Module (ECM) monitors the temperature of heated oxygen sensors 1 (Pre-Catalyst O2 sensor) and heated oxygen sensors 2 (Post-Catalyst 02 sensor).


A three way catalyst converter like the one shown in the image has a high oxygen storage capacity. The oxygen sensor compares the oxygen inside the catalyst against the oxygen outside the catalyst, before entering the catalyst and after it exit. This oxygen measure is read by the ECM as a voltage. The ECM simply compares one sensor to another (frequency ratio) if this ratio approaches a specific limit value, the three way catalyst malfunction is diagnosed.

What are the symptoms of code P0425 ?

The symptoms presented are Check Engine light ON or Service Engine Soon Warning light. In some vehicles the ECM will trigger “Limp” mode which will adjust the air/fuel mixture richer to protect the engine. This “limp” mode will cause your vehicle to consume more fuel.

In this case the repair importance level is moderate, because of this problem the vehicle will more than likely fail the emissions test and may affect engine performance or gas mileage. The repair difficulty level is also moderate due to it could require replacing a component (sensor, solenoid or switch) and clearing the engine code.

How do you troubleshoot code P0425 ?


As the code description implies, it means that the vehicle’s control module has detected that the three-way catalytic converter is not working properly, which means is not as efficient as the factory is expecting. The possible causes for this code are three:

  • A faulty Catalyst Temperature Sensor

  • Catalyst Temperature Sensor circuit poor electrical connection

  • Catalyst temperature sensor harness is open or shorted

Now that the causes for this code are known, at first you should check for exhaust leaks (particularly before the catalytic converter), repair them as necessary, reset the code, and reset the system. Several tools may be needed to successfully diagnose this code if no exhaust leaks are detected. Check all electrical connections that involve the Primary Oxygen sensor, sometime due to heat and vibration the electrical connector may come loose and the Check engine light will come on. If that is your case, assure good electrical connection and clear the code. Go for a 15 min drive and if the Check engine light doesn’t turn ON again you’re good to go.

Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data. Proceed by raising the vehicle and pointing the temperature gun at the exhaust pipe before and after the catalytic converter in question (this is much easier if the temp gun is equipped with a laser pointer). Compare your results with the manufacturer specification, if your results do not coincide with what the manufacturer recommends, then the catalytic converter is most likely bad

After checking the electrical connection, the next step is to check the Catalyst temperature sensor harness. It is important to observe the cables present no burned signs, torn out cables, or extremely loose cables that can hit any moving component while the vehicle is moving. One way to check the sensor harness is taking a multi-meter and determine to which ECM module the sensor is connected, once you found it, you should search for your model specific electrical schematics, and check continuity between the lead that is connected to the oxygen sensor, and the ECM module lead. This method is 100% effective to check if the electrical harness involving the Catalyst Temperature Sensor, but it can be tricky to obtain the schematics.

At last you should check the operation of the sensor, before you can test the operation of the sensor, you will need a multi-meter and if possible an oscilloscope. You should first check that the basic engine setup is to the manufacturer’s specification, then thoroughly warm up the engine, remember that the sensor will only function once it has reached its operating temperature.

The two methods involve testing the oxygen sensor using and oscilloscope or a multi-meter. An oscilloscope is the best method for testing. This will give you the exact output of the sensor along with its response times. A multi-meter can also be used but this will only give an indication if there is an output or no output. The sensor switches frequency too quickly for any response time to be measured.


Oscilloscope Method

Connect the sensor output to the oscilloscope, and run the engine at approximately 2000 rpm. If you can see in the oscilloscope a fluctuating voltage between 0.1 and 1.0 volts, and you see a quick response time (approx. 300 milliseconds), this means you have a properly functioning oxygen sensor and you should replace the Catalyst Temperature Sensor.

Multi-meter Method

Connect the sensor output to the multi-meter, and run the engine at approximately 2000-2500 rpm. The output will be a DC voltage oscillating between 0.1V and 1.0V. Some sensors may require to be set on AC voltage measurement to correctly read the sensor output. If the sensor output is constant or the response time is too slow that means you have a bad oxygen sensor and a good Catalyst Temperature sensor, you should be replacing the oxygen sensor.


Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic converter needs to be replaced to fix the problem. If the P0425 code is combined with other codes, try fixing the other codes first.

The most common misdiagnosis is caused by not thoroughly investigating what led to catalytic converter failure. Catalytic converter failure occurs when other codes are present and left unattended for long periods of time. This related codes may be:

  • P0422 Main catalyst, Bank 1 Below Threshold

  • P0440 EVAP Emission Contr. Sys. Malfunction

  • P0452 Evaporative Emission Contr. Sys. Press. Sensor Low Input

  • P0453Evaporative Emission Contr. Sys. Press. Sensor High Input

It’s important to consider this are not the only related codes that may appear with this particular issue. Remember to diagnose and repair oxygen sensor codes, fuel trim codes, fuel mixture codes, or misfire codes first before attempting to diagnose a catalytic converter code.

Engine misfires are also known to deteriorate the platinum element of the catalytic converter, as is excessively rich exhaust. Oxygen sensor failure should be verified before replacement. Sometimes techs report that oxygen sensors are often replaced blindly in order to avoid costly catalytic converter. This just leads to added expense. Also, aftermarket and “rebuilt” catalytic converters have proven problematic. Although they may cost much less, they provide neither the efficiency nor the longevity of OEM quality catalytic converters.

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