2002 Mitsubishi Galant Front Catalytic Converter

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Hi there:DTC P0421 - Warm Up Catalyst Efficiency Below Threshold (Bank 1)


Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.


You will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.



A code P0421 may mean that one or more of the following has happened:



The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
A sparkplug is fouled up


Possible Solutions: Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.


One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.


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Solenoid is your problem, the solenoid is connected to the (EGR) exhaust gas resirculation, when the solenoid fail, the EGR don't work right, and your O2 sensor on the cat is reading to much exhaust gases in the cat, so your catalytic converter is failing, solution go to a junkyard and get one from other engine or similar the looks clean and replaced, run your car for more than 10 miles and your service soon engine light should go off, if that don't append your O2sensor on the cat is no good.

P0421 - Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms: You will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.

Causes: A code P0421 may mean that one or more of the following has happened:
The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
A sparkplug is fouled up

Possible Solutions: Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.


hope this helps (remember to rate and comment this answer).

this code relates to catalytic converter efficiency, could be a bad o2 sensor, bad cat , prob gonna need a mechanic to diagnose, if you do decide to replace o2 sensor you want to change the one behind cat, tghis one monitors cat function

I believe this has to do with your catalytic converter. Sounds like it might be getting old. The older they get the slower they are to respond. this is due to contamination. Same is true for your Oxygen sensors.

P0421 is Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Causes:
A code P0421 may mean that one or more of the following has happened:

• The catalytic converter is no longer functioning properly
• An oxygen sensor is not reading (functioning) properly
• A sparkplug is fouled up

Possible Solutions:
Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers
offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into

Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions sys,,,,,, Causes A code P0421 may mean that one or more of the following has happened:

• The catalytic converter is no longer functioning properly
• An oxygen sensor is not reading (functioning) properly
• A sparkplug is fouled up ,,,,,, Possible Solutions Measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.
  One thing to note is that many vehicle manufacturers
  offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.
  

DTC code P0421 is the O2 sensor that indicates a an issue with the Catalytic Converter. Replace the O2 sensor to the Catalytic Converter "the one after Converter" It's common for the sensor to fail and you can pick one up from Auto Zone and they will have the step by step instructions to replacing the O2 sensor. 
DTC code P0302 Indicates a misfire on cylinder #2. Address the spark plug, spark plug wire, and injector. Look for any damages and try running some fuel injector cleaner through the fuel system to clean out the injectors. 
Good luck and hope this helps. 

I have the same vehicle, same code P0421, changed the BOTH O2 sensors and the check engine light was still on, took it to my fathers garage and checked the Catalytic Converter, and it was ok later I found out that P0421 means that the vehicle doesn't have the proper mixture of gas and air (running richer) so I changed the MAX air flow sensor and wala the code went away.