Just posting my experience in troubleshooting PO420 (Cat Inefficiency code), from start to finish, and common obstacles along the way. I have a 1999 3.4l 4Runner with 137k:
It started with PO172. I cleaned the MAF with carb cleaner, chased that with rubbing alcohol. Then I cleared the code, and it never came back. Acceleration was noticeably more responsive and powerful.
50 - 100 miles later I get a check engine light CEL, but now the code is PO420. After researching this here and on other forums, I decided to troubleshoot from least to most expensive $, resetting the CEL after each step. General idea is get everything running as "clean" as possible.
1) Change air filter, oil and oil filter (duh). Reset CEL. CEL came back on after about 50 miles.
2) Change PCV valve and fuel filter, ran some Lucas injector cleaner thru the fuel system. Reset CEL. CEL came back on after about 50 miles.
3) Next step is replacing the sensor(s), and this is what solves the problem in most cases in my research. Replacement cats are typically
not necessary.
Replacing the AFR and Oxygen Sensors
From what I can tell, most if not all '99 and later 4runners are California emissions vehicles, regardless of whether they're from or have even been to Cali (yeah I said "Cali", who cares?) So there are 2 cats and 2 sensors. It should be obvious from that, but you can double-check by reading the specs directly under the hood. Also, the dealership can tell you based on VIN.
From front to back, the 1st sensor is the Air/Fuel Sensor. It is on the bend before the 1st cat. The 2nd is the o2 sensor. It is after the 2nd cat. You need to buy sensors from the dealership, or Denso. Don't use Bosch. Best bet is Denso sensors from Amazon.
DO double-check to make sure you are getting the right sensor(s) for your vehicle. Never modify the wrong sensor to make the connection fit, etc. First call your Toyota dealership for the Toyota part number, they will check by VIN (their prices are 2-3x what you'll pay for Denso on Amazon). Then cross-reference that with Denso here:
DENSO Aftermarket Cross Reference Online Catalog Also check here:
Oxygen / AFR Sensors - Underdog Racing Development
Also buy replacement nuts (M8 x 1.25), washers, and maybe some M8 x 1.25 15mm long bolts, because there is a chance your studs will break. Also, make sure you have some anti-seize on hand.
When you're ready to replace the sensors, start out by saturating the old sensor nuts with PB Blaster, tap the studs to help work the PB thru the threads. Let it soak, and repeat a few times, for at least a few hours, or a couple days if possible. The original nuts are 12mm sockets I believe, but they're probably so rusted, you'll need small sockets, or a variety of pliers and vice grips to get them off. Careful with vice grips, that's how I broke a stud.
If you don't break a stud, follow the instructions for installing the sensors that come in the box. Apply a good amount anti-seize to the stud, put on the new washers and nuts, tight but not overtight (it's only a sensor, and again, you don't want to break a stud), cover the nuts again with anti-seize. Reset your CEL, and you should be good.
If you do break a stud, if you're relatively lucky like I was, enough of the stud will be exposed for you to unscrew from the bung using vice-grips. This is a strong man challenge. After two days of working at it with vice grips and PB blaster, the sucker turned!!
:girl:
I ended up getting it with needle nose vice grips (down in the plier where they mouth opens a little wider, if that makes sense), with a standard set of vice grips gripping the needle nose vice grips even tighter on the stud to keep them from spinning on the threads. Warning: Make sure you wear eye protection; your tools could break doing this. Then with all your strength clamp those suckers on and start turning. I'm a pretty strong dude, and this was hard, but I eventually got. Replace the broken stud with an M8 x 1.25 bolt, anti-seize the crap out of it.
If you break a stud but can't get it with vice grips, you might be lucky enough to access with a drill and easy extractor kit, or maybe you can tap it out and use a slightly larger size bolt. Otherwise you will have to drop out that section of the exhaust. I have no experience to report on that (thankfully), good luck if that's where you land.
That's all I got, hopefully this helps somebody out.
