I am going to try and be as detailed as I possibly can so this is going to be a long one! I just got this 2011 Honda Pilot with 172186K miles from a private seller and I knew that there was a P0420 code when I bought it because I brought my OBD2 scanner with me and he had told me he got the 02 sensors replaced so I knew it was likely going to need a new CAT. It drove completely fine and the check engine light was NOT on yet. I asked what maintenance he had done and he said he had the following recently replaced:

• O2 sensors
• H20 pump
• Rear shocks
• Front/rear tie rod ends
• Alternator
• transmission Flush and service

A few days later (end of march) the Check engine light came on but still drove fine. I went and got a full synthetic premium oil change, new oil filter, cabin air filter, and fuel system cleaner and the check engine light turned off for about a week then came back on.

at the end of May 2022, I brought it to the Infiniti dealership to get the CAT replaced because I have a friend who works there and he offered to help. The receipt from the dealership says they replaced the following:

• Catalytic Converter (California compliant)
• Exhaust Gasket
• Sway Bar Link
• RH Lower Control Arm.

After that, it drove fine without any issues and no Check engine light. at the end of July I took it to get emissions done and I passed but a few days prior I had started noticing that the car felt like it was ever so slightly jerking sometimes while driving, when i put it in reverse or into park it felt like a slight thud/shifting hard, I starting getting bad gas mileage and when I put the key in to start the car, I had to put the key in and turn but wait 4 seconds before turning it all the way on otherwise it would take a few seconds to start. I asked my friend and he said it sounded like the starter was going out but I didn't bring it in because it wasn't happening that much and wasn't that dramatic when it did happen.

Last Thursday, October 14th, 2022 I was driving on the highway and the check engine light came on so I decided to get off the highway and plug the OBD2 scanner in and it had the code P0420. I will put the live data information below to see if that helps at all. I had been driving for maybe 10 min before stopping to plug the scanner tool in. when I started driving again I noticed it started jerking more and just felt way worse than before and my gas mileage went wayyyyy down.

I brought it in to the dealership Monday Morning and he said that the only code it's showing is the p0420 and it's specifically referring to the CAT they replaced. I asked if there was any way at all that it could be due to the o2 sensor or needing to change filters or something and he said no and that he would call me later when they know more. He called just before they closed and said that there is too much air getting through and I would need to replace the head gasket which is very expensive and might not even fix it because ultimately the engine is going bad and needs to be replaced and suggested I trade it in.

Everything I have looked up says that it should have been fixed after he replaced the cat and the exhaust manifold and it couldn't be a bad engine because I would have failed emissions. Does anyone have any clue what the heck this could be? Did they just put the wrong cat in or not place it properly? Any insight or advice is super appreciated!!

Here is the live data and IM readiness from my scanner tool:

LIVE DATA
DTC_CNT: 1
FUELSYS1: N/A
FUELSYS2: N/A
LOAD_PCT(%): 0.0
ECT (farenh): 180
SHRTFT1(%): 0.0

LONGFT1(%): 4.7
SHRTFT2(%): 0.0
LONGFT2(%): 4.7
MAP(inHg): 25.1
RPM( /min): 0
VSS(mph): 0

SPARKADV: 0
AIT( Farenh): 100
MAF (lb/min): 0.040
TP(%): 17.6
02B1S2 (V): 0.030

SHRTFTB1S2(%): 0.0
02B2S2 (V): 0.090
SHRTFTB2S2(%): 0.0
RUNTM(sec): 0
Mil_Dist (mile): 3

EGR_PTC(%): 0.0
EGR_ERR(%): 0.0
EVAP_PCT(%): 0.0
FLI(%): 32.5
WARMUPS: 167
CLR_DIST(mile) 3626

EVAP_VP(inH20): -0.2
BARO(inHg): 25.1
EQ_RAT11: 1.999
O2S11(mA): 0.020
EQ_RAT21: 1.999
O2S21(mA): 0.020

CATEMP11 (f): 102.20
CATEMP21 (f): 102.20
VPWR(V): 11.325
LOAD ABS(%): 0.0
EQ_RAT: 0.999
TP_R(%): 8.6
TP_B(%): 34.1
APP_D(%): 19.6
APP_E(%): 9.4
TAC_PCT(%): 6.7

I/M Readiness since DTC Cleared:
MIL Status: ON
Misfire Monitor: OK
Fuel System Mon: OK
Comp Component: OK
Catalyst Mon: OK
HTD Catalyst: N/A
EVAP System mon: OK
Sec Air System: N/A
A/C Refrig Mon: N/A
Oxygen Sens Mon: OK
Oxygen Sens her: OK
EGR system: OK

I/M Readiness This drive cycle:

MIL Status: OFF
Misfire Monitor: OK
Fuel System Mon: OK
Comp Component: OK
Catalyst Mon: INC
HTD Catalyst: N/A
EVAP System mon: N/A
Sec Air System: N/A
A/C Refrig Mon: N/A
Oxygen Sens Mon: INC
Oxygen Sens her: INC
EGR system: INC

 

2011 Pilot has three catalytic converters.

Each cylinder head has its own converter, these are the two monitored cats that can cause the p0420. The third cat is down stream and is not monitored.

Did the cats directly on the cylinder heads get replaced or was it the 3rd cat that was replaced?

 

The mechanic I took it back to after the code came back said it was that specific cat they replaced in bank 1.

 

Is it burning oil?

 

I just looked up what that means and I haven’t been told it’s burning oil and haven’t smelt anything burning, nor seen any smoke at all. I’ve gotten two oil changes since I bought it. Both full synthetic high mileage. I’ve taken really good care of it since I’ve had it.

the weird thing is, the check engine light just went off by itself yesterday afternoon and I didn’t do anything to it. I don’t know if this helps with diagnosing it or not? I took it to another mechanic and he said my engine is fine, it’s just a bad cat. I don’t know what would cause a new cat to fail after 2 months

 

Sometimes, it is just the wrong catalytic converter replaced.

 

A misfire can cause this along with stumble. What brand or pic of coils?
are you losing any coolant? Open cap when cold, is the coolant full to the cap?
I wouldn’t flush transmission, rather dump and fill.