Multiple error codes (P300, P301, P303, P420, P430)

[agr00m]

This is my sister's vehicle. Her CEL came, so she took it to a local part store which came back with the following error codes and quoted her over $2,300 for a new catalytic converter (both banks). I put new wheel bearings on over the summer, and she had the water pump replaced last month for over 2k, so is asking if I can help.

• P0300 - Random or Multiple Cylinder Misfire Detected
• P0301 - Cylinder 1 Misfire Detected
• P0303 - Cylinder 3 Misfire Detected
• P0420 - Catalyst System Low Efficiency - 13.2%
• P0430 - Catalyst System Efficiency Below Threshold (Bank 2)

The install looks pretty straight forward, so my question is whether the failing catalytic converter is causing the misfires, or are the misfires causing the catalytic converter to fail? Or is it just one big negative feedback loop, each accentuating the other? I'd hate to replace the CC, only to have the misfires continue and prematurely start to deteriorate the new CC.

What I also find curious is that cylinders 1 & 3 are on bank 1, yet the P0430 code is for bank 2. And does the P0430 code mean only the bank 2 CC would need to be replaced?

Finally, should the studs/nuts be replaced when replacing the CC? Watching a few videos, some do, some don't.

Thanks in advance!

 

[agr00m]

Sorry, should have noted it's a 2017 LTD, 3.5L Ti-VCT 4WD.

 

[J_C]

Welcome to the forum!

I wouldn't yet assume that it needs new cats, but it is true that running long enough with misfires can damage them. Typically if the vehicle is no longer driven once it has the misfires often enough to set the code, then the cats are still serviceable.

How many miles on it and has routine maintenance been done like replace the spark plugs by 100K mi?

I'd hook up a scan tool capable of live data and look at the fuel pressure and the O2 sensor readings, and long term fuel trims.

A failing catalytic converter will not cause misfires, unless it is clogged. You can remove the O2 sensor and check the exhaust pressure, but I think not clogged because not all cylinders on that bank (into same cat) are having the issue.

I'd go ahead and replace the O2 sensors before I'd go all out and replace the cats... since if I were replacing the cats, I'd want the new O2 sensors anyway, and yes "IF" the cats are damaged, it is probable an upstream problem caused that (or rust-through which generally, you'd hear) and that problem should be corrected before the new cat(s) are installed.

It might be time to take it to a good (and trustworthy) mechanic. Parts store diagnosis based only on OBD2 code pulling is not reliable.

Pulling the engine to do the water pump is a fairly invasive procedure so I am wondering if this happening so soon after, is related, possibly some wire damage, connector not fully plugged back in, or something.

 

[agr00m]

Thanks for the reply. It has just over 180k miles. No idea about the maintenance, but knowing her, probably not.

I'd started to come to some of the same conclusions and decided to check the plugs & coils before doing anything with the cat. I had not considered a wiring issue from the water pump replacement though or O2 sensor.

It might be time to take it to a good (and trustworthy) mechanic. Parts store diagnosis based only on OBD2 code pulling is not reliable.

Hah, I've been telling her this from the start, but she's afraid once it's in the shop they're going to stick her with a huge bill she can't afford. She also a single mom of 2 and lives 1.5 hrs from me with a crazy work schedule (service industry) making it even more difficult.

Will probably go there this weekend though and start with a set of new plugs all around and see if I notice any wiring issues. If the misfires still occur, maybe replace one of the coils and/or the O2 sensor and see where that gets me.

Appreciate the help!

 

[peterk9]

A very late Welcome to the Forum.

Peter

 

[agr00m]

Thank you @peterk9

I should follow-up by saying that we ended up replacing the two ignition coils (1 & 3) and all 6 spark plugs, and so far the error codes have not come back.

 

[agr00m]

Spoke too soon. The P420 & P430 codes came back this morning (but not the P030x codes). So it seems we fixed the misfires, but not the Catalyst System Low Efficiency.

I told her we could replace the O2 sensors next, but she said they were all replaced earlier this year. I questioned whether it was really all 4, so she's double checking. But if she's correct, I assume our next step is replacing the bank 2 CC.