Re: Code P0420 Bank 1: Which O2 Sensor Do I Replace?
FJT:I still get the feeling that you're shot-gunning this problem, don't have a good enough understanding of the OBDII monitoring system to properly interpret the code data, and are moving a little too quickly on the expensive cat replacement.
For instance, you originally said you had a "random misfire" code, and "corrected" it by cleaning the MAF. In the absence of other severe engine problems, a dirty MAF would almost never be capable of causing a random misfire.
LT: I must be mistaken. I was under the guise that the MAF is what tells the ECU what the air/fuel mixture should be based on ambient conditions. If the car is running too lean or rich wouldn't the ECU read that as a misfire? It was code P0300.
FJT: Incorrect. The MAF sensor (mass airflow sensor) ONLY provides information on the mass of air molecules passing through the throttle body, irrespective of air temperature or absolute pressure (altitude). The engine ECU takes the output from the MAF, the engine coolant temperature sensor, the intake air temperature sensor, both channels of the throttle posiion sensor, the exhaust air-fuel sensor, and multiple other sensors to determine the amount of fuel that needs to be injected to maintain the desired 14.7:1 air-fuel ratio. If ANY of these sensors are providing erroneous outputs to the ECU, the air-fuel ratio may be incorrect.
The P0300 fault code indicates that a random/multiple cylinder misfire was detected. This is usually related to an ignition system fault, not a mixture control fault. However, defective spark plugs, defective ignition coil(s), defective crankshaft position sensor, defective/damaged wiring, incorrect valve clearance, defective head gasket, could all be factors to a P0300.
And you thought that replacing the gas cap could possibly have an effect on a P040 code?? A leaking gap cap seal would trigger an OBD code relating to the evaporative emissions control system, NOT the fuel feedback system or catalytic converter efficiency.
LT: There are only a hand full of possibilities with code P0420. Using process of elimination, both sensors have been replaced and the code still comes back on. After analyzing my actions, the only delta that has occurred in between the CEL coming on is after refueling. Which is why I decided to give the gas cap a try. It's a $10 risk I was willing to take.
FJT: Incorrect, there are MANY factors and conditions that can trigger or contribute to a P0420 condition. Regardless, the fault code associated with a loose or damaged gas cap would be an evaporative emissions fault, not a misfire or catalytic converter efficiency code
What are you going to do if the P040 code comes back 2-3 driving cycles after replacing the cat?
LT: Since both sensors have been replaced there can only be two possibilities left.
1) Bad Cat
2) Leaky exhaust
If the gasket between the manifold and the block is weak it could be introducing air into the system causing the sensor to
trigger the code
If I replace the cat, the gaskets will be replaced as well. This should eliminate all possibilities, unless I should factor in vacuum leak. [/B]
FJT: OMG no, as stated above there are literally at LEAST a dozen more additional faults or conditions that can affect mixture control and trigger the codes associated with catalytic converter efficiency.
There are MANY more diagnostic tests, measurements and evaluations that you should do before spending $1,000+ on replacing a "suspected" bad cat.
LT: I’m open to other tests and happy to try them. Can you please provide examples?
Providing a complete working knowledge of how fuel-feedback systems work, and how to troubleshoot them is far beyond the scope of what can be accomplished in a few forum posts. I suggest that you search for "factory service manual" on this forum to see where you can download the complete (180 meg) 2007 FSM. Or, look at the "Technical Documents" section at the PureFJCruiser website and download the relevant sections of the manual from there. There is a tremendous amount of information in the manual on the Emission Control System, the Sequential Fuel Injection System, how everything works, how to check sensor outputs, etc. At a minimum you need a an OBDII code reader, a high-quality digital multimeter, and for some measurements you will need a lab oscilloscope or other type of diagnostic tester.
Lastly, did you run a CARFAX before purchasing the vehicle, or look at any other documentation that would verify the indicated mileage? If the cat is truly bad, I would suspect that the 89K mileage shown on the odometer is not correct.
LT: The CARFAX came back clean and passed emissions every time. Based on my research, every thing P0420 related was either resolved with replaced sensors or cat. I did notice that there's corrosion in the pipes. Could that have caused it to go bad? I've owned cars with more mileage and none of them had cats that went bad. The only thing the owner did different was take it off roading or water crossings.
FJT: Off-roading or water-crossing should not have damaged the front cats. Again, I suspect that your cats are fine, and that your "low cat converter efficiency" code is caused by some other fault that you have not yet isolated.
Since the VERY LAST thing you did before the CEL came on was to change the air filter, re-install the OEM filter and see if that has any effect. Normally it shouldn't. but as part of the troubleshooting process you always "reverse" any recent changes that were made immediately prior to a new problem appearing.
Last edited by FJtest; 09-21-2018 at 01:50 PM.