Need help, P0420 and P0300

Hi, I have a 2006 Tacoma and had the check engine light come on with codes P0420 and P0300, vehicle has 187634 miles on it, I'm hoping that I don't need a new catalytic converter, any help greatly appreciated

 

I would start with o2 sensors and checking spark plugs.

 

Thanks, will be checking that in the morning, up stream or down stream?

 

p0420 is common when the gas cap is not sealed. Do you have an aftermarket gas cap? If so, that can cause that code. If it is OEM gas cap, make sure it is tight and make sure the gasket is not cracked and worn out. As for the 0300 code, I would clear it and see if it comes back. When were the spark plugs changed? If it has been a while or if it has not been done, that would be my first guess. I don't think it would be O2 sensors, as those typically throw a different code. I can't remember what code it is, but there is a code specifically for the O2 sensors if I recall correctly.

 

As cheap as they are I would just replace all of them. As mentioned about also check gas cap. I wouldn't rule out a head gasket failure as well. Run a leak test and compression test to rule that out

 

@irayfz6, how can a loose or bad gas cap cause a catalytic converter efficiency code

 

I would love to explain, but I don't know the answer. However, I have seen this code on three different vehicles, and in each case it was the gas cap. Two of them had aftermarket locking caps, one had a very old OEM cap with a dried out, cracked gasket. When I couldn't diagnose my issue the first time I encountered the code, I turned to forums and read somewhere on a thread that it is common for a loose gas cap to throw that code.

 

Gas cap codes are p0455 (gross evap leak) not p0420 (cat effic below threshold).

Don't change 02 sensors either, they are working and are telling the ecu that the catalytic converter has failed on bank 1. Very rare that bad o2 sensors can trigger p0420 because they need to work to show the rear sensor swinging with the upstream.

For me, I would go after the misfire first. A serious misfire can cause raw fuel to hit the catalyst and wreck it. Check plugs and wire condition first, inspect cylinders and see if any are washed with fuel.

 

I got a P0420 when I was out wheeling one day after filling up at a 76 station like I always do (different station though a long ways from home). Cleared the code on the trail and it popped back up right away. Gas cap was tight (first thing I checked). Went home, cleared it and filled up again and its stayed off for a few months now, even on other wheeling trips. Checked my OBDII scanner and no codes. Chalked it up to some funky gas but I probably just got lucky there.

 

Thanks all for your help, I'm beginning to lean towards a bad tank of gas, on other vehicles that I've seen through a P0420 they had drivability issues with the code, I don't have any drivability issues at all, I'm also beginning to wonder if the random misfire was from bad gas

 

Same here. Mine drove fine other than the code that kept popping up while I had that tank of gas. Hasn't come back since, hopefully your's is the same. I'd run some Lucas Fuel system treatment through the tank a few times as well for good measure (or Seafoam or whatever floats your boat).

 

Brian, these are super serious codes and can only be cured by upgrading to a 3rd gen stack.


So sorry...

 

Hell no, I think I may have just figured them out, hooked up my friends factory Toyota scan tool, and found some other codes, he told me to start with an oil change, I know I'm about a 1000 miles over due at the most, but money's been tight, found codes for timeing, and the VVT works off of oil pressure, so he thinks there's not enough oil pressure

 

Well, I cleared the codes, changed the oil, and added a bottle of Heet to the fuel tank, been driving around all day and so far no check engine light

 

Nice

I would have never guessed oil.

 

Neither would I, but hooked up my buddy's factory Toyota scan tool, and found other stuff, and looked at some live data, the VVT system works off of oil pressure so, I last did an oil change in February, so by now the oil was starting to degrade and couldn't supply enough pressure to the VVT system, so it was throwing the ECU off