An Update... Ended up swapping out the Coil drove about 400 miles.. no misfire pop up ... yet. So back to the P0421 code... I hooked up my torque app ODBII reader.. Bank 2x2 is reading at .8v at idle, and it'll mirror 2x1 graph when the RPM is at 2500 which I believe that sensor is bad? Are you guys replacing this precat? Which is Bank 2, the sensor 2x2 is the one below near the radiator. But the code P0421 is near the firewall?? which one is bad?
Just for the record. A P0431 and P0421 are both warm-up catalyst (pre-cat) related. This code will come up if the downstream O2 sensor, the ones after the warm-up catalyst (#1 & #3) fluctuates voltage too quickly
How it works:
The O2 sensor monitors the hydrocarbon level of the exhaust and adjusts the fuel mixture accordingly depending on if it is too rich or lean. This is why that O2 sensor (both upstream B1S1, B2S1) will fluctuate voltage wildly between 0.1 (lean) - 1.0 (rich) Volt. the downstream oxygen sensor (B1S2, B2S2) will measure hydrocarbon after the catalyst to see if it is functioning. This downstream sensor should read a relatively steady voltage. If the voltage fluxuation is looking the same as the upstream sensor (moving widely between near zero and one volt) then the PCM (computer) gives you a P0421 or P0431 code after being logged on two consecutive drive cycles.
1 = Bank 2 warm-up catalyst (pre-cat)
2 = Bank 2 TWC (three-way catalyst) catalytic converter
3 = Bank 1 warm-up catalyst (pre-cat)
4 = Front pipe (flex pipe)
5 = Bank 1 TWC (three-way catalyst) catalytic converter
6 = Presilencer (resonator) - not pictured on OEM exhaust but would bolt directly downstream from bank 1 TWC.
The P0421/P0431 is almost never the fault of a faulty O2 sensor. As sensors age and reach the end of their lifespan (50,000mi for a 2 lead, 70,000mi for a 3-lead, 100,000mi for a 4 lead) or (0.1V-1.0V response > 300 milliseconds) they tend to respond slower. This would reduce your effective regulation of mixture and also slow the response telling you that a cat was bad. this would mean you are less likely to show a P0421/431 code but more likely to destroy your catalyst due to improper fuel mixture regulation.
If your pre-cat/warm-up cat/manifold goes bad it is most likely a result of a long standing misfire (P0300.301,302,303...etc) This cat will disintegrate due to prolonged overheating (what gives you the sulfur smell) and travel downstream to clog the rear catalyst (if it does not also get sucked back into the intake via the EGR).
If you replace the front catalysts only then the increased back pressure from clogging/decaying rear catalysts will destroy the front ones within 20-50K miles (estimate)
The only solution that results in a correct long term fix is to:
1. Eliminate the misfire (usually COP's and Spark Plugs x6 in the Mazda 6 3.0 v6)
2. Replace all 4 catalytic converters. (Bank 1 and Bank 2 warm-up catalysts and TWC)
3. Replace all 4 oxygen sensors
In 2017 the total cost of parts for this job is as follows:
MFG / Part # / Desc.
464 16413 - Manifold/pre-cat
464 16414 - Manifold/pre-cat
EMI 40627 - Catalytic Converter
EMI 40626 - Catalytic Converter
NGK 22097 - O2 sensor
NGK 22118 - O2 sensor
NGK 22100 - O2 sensor
NGK 22115 - O2 sensor
037-8102 - Exhaust Manifold Gasket Set
673 6005 - Ignition Coils x6
AJTT-18-110 - Spark Plugs x6
9XG01732S - manifold studs (x12)
9XG0170602 - manifold nuts (x12)
Total Cost : $1379.03 USD
(price also included engine air filter, oil filter and replacement rear wiper blade)
Total labor time is about 6-8 hrs (working on the floor)
unless you have a broken manifold stud. Then add 20 hrs +/- (the v6 likes to break off the bank one above the alternator)
In order to remove the section it is usually necessary to unbolt everything except the manifolds (warm-up cats) and starting forward of the presilencer (resonator) and continue forward.
Taking off the two 8mm bolts from the power steering shield and removing it will help with clearance.
If you need more space to access the bank 2 TWC then you can remove the bolt from the passenger engine mount and jack up the motor a small amount.
The difficult part is removing the front pipe/flex-pipe. This needs to be lowered from the rear and pivoted towards the driver-side. While lowered and pivoted it needs to be rotated anti-clockwise (from the perspective of facing forward)
I was able to lay under the car with my feet towards the rear. I would support the back of the flex pipe with my knees while I rotated and worked the 180 degree bend up and over the steering rack.
If this is too difficult then you could remove the 4 bolts and 2 nuts from the cross-member rear support plates and move the entire crossmember/steering rack/engine (use a support) down a few inches.
Lowering the crossmember too much without disconnecting the 3 steering rack bolts will damage the rack (very expensive).
Pay attention to how it finally comes out because it will have to go in the same way.