Hello, I got P0430 (catalyst efficiency below threshold) 2 days after the spark plug change. I changed the spark plugs but didn't change the coils. OBD2 scanner can't detect any misfires, but sometimes RPM is not constant at idle. Also, I observed a 25% decrease in MPG. The mileage is 115K. What would you suggest? Do you think it's a coincidence or spark plug caused the P0430? Is it possible to have misfires that are not detected by the OBD2 tool?
P0430
This DTC can be triggered by a faulty catalytic converter or oxygen sensor, which can be caused by an engine misfire or rich/lean air-fuel ratio.
You would want to do a vacuum test at idle to check to see if the converter is restricted. If it is restricted the vacuum reading will slowly decrease when at a hot engine idle speed
Otherwise at 115,000 miles, the O2 sensors are not sending signals fast enough to the PCM. So, the PCM is making fuel trim adjustments that are not current for the engine conditions because the O2 sensors are worn and the O2 signals are too slow. Replace all of them.
As a post script all engines misfire. Assuming the definition of misfire means 95% of the air/fuel mixture is burned is all cylinders at all times under all conditions. The OBDII system is only going to generate a code based on a misfire IF the misfire is great enough to not more the crankshaft a certain number of degrees of rotation and that occurs several times in a minute or so.
The spark plugs did not have much to do with the P0430 code.
The issue is more related to air/fuel mixture, likely being too lean.
It's strongly recommended to use the correct Motorcraft plugs in the 3.5 EB. Put the original plugs back in for now to see if things go back to normal.
As for the P0420/P0430, it's rare the cause is actually a bad catalytic converter. If you've experienced a flashing CEL, and kept driving for a while during a misfire condition, that could kill the cats. During a misfire, fuel is basically dumped into the cats, as the cylinder(s) can't burn the fuel being delivered. There are multiple things that can trip those codes; quality/quantity of oil, coolant and fuel; the smallest of exhaust leaks, possibly o2 sensors, and the list goes on.
If you recently purchased gas from somewhere unfamiliar to you, it could be bad gas. If putting the old plugs back in doesn't change anything, try topping off with known good quality PREMIUM gas, and get a can of Cataclean to dump in the tank. I used it once in a Lexus that popped the P0430 code due to junk gas, and the problem went away after doing what I just recommended.
As a post script all engines misfire. Assuming the definition of misfire means 95% of the air/fuel mixture is burned is all cylinders at all times under all conditions.
the definition of misfire means less than 95% of the air/fuel mixture is burned in all cylinders at all times under all conditions. That kind of thing happens all the time
And know the OBDII system does not consider the above definition for a mis-fire condition. To generate a mis-fire code, the crankshaft has to rotate LESS than expected after a spark plug is fired off. And the event has to do that multiple times which is when a code is generated and the CEL comes on
