OBDII Code Help

2002 Tacoma, 3.4L Auto 280k miles... On the way home this evening, my truck started running very rough under load. No MIL for about 15 minutes. The MIL did come on. At home I pulled 4 codes:
P0300 - random multiple misfires
P0303 - Misfire on #3
P0306 - Misfire on #6
P0420 - Cat efficiency below threshold

All codes at the same time. I cleared the codes, swapped the #1 coil and #5 coils. Test drive... Less than 4 miles all 4 codes are back. Misfires have not changed locations.
Cleared codes, swapped #3 and #6 wires with another vehicles wires. Test drove... Exact same results. Less than 4 miles all 4 codes set again.

Wires are OEM replaced 2 years / 50k miles ago (eliminated by the swap)
Plugs are OEM also replaced 2 years / 50k....
Coils are 5 years / 120K ago (eliminated by the swap)

Bad Cat?

Who has a thought?

 

Bad cat would be my guess.

 

i would guess either an intake, exhaust, or head gasket has gone bad. try switching your knock sensors for shits and giggles IF you have more than 1.

 

Check your intake. Might be a bad gasket at the heads. Brother had same issue on his GMC truck. He pretty much replaced every thing up top, ie.injectors, distributor, plugs, wires. Turned out it was a bad intake gasket on driver side head. Also if you can get a pulse injector tester and gauge. Do a leak down test on those injectors plus one not misfiring for reference. Look up on Youtube.

 

The misfires are causing the PO420 code from the unburned gas being dumped into the exhaust

What did the number 3 and 6 plugs look like ??

 

Sounds Like you have a cracking valves leak down test would be your best shot