Vehicle: 08 V6 T4R with 135000 miles.

So I had my check engine light, traction control, and (flashing) VSC lights all pop on last night and they are still there this morning. When I checked the codes, they came out to be P0430 and P0420 (Cat System Efficiency Below Threshold).

From looking around on the forums, it looks like this is either my cats (obvious) or O2 sensors. I'm not sure if the O2 sensors have ever been replaced, TBH.

A few questions:

1) Would trying to replace my O2 sensors be the best course of action?
1a) If so, replace all four?
1b) Are these the correct part numbers: Denso 234-9051, Denso 234-4261?

2) Why is my MPG suddenly BETTER after these lights came on? I'm talking going from 15ish to around 18/19.

3) Most importantly: are the VSC and TRAC actually disabled? As in, do these lights being on pose a safety hazard?

Thanks for the help, forum guys.

I'll address your questions in reverse order, going from the simple to the more complex.
3) Almost anytime the ECU detects a malfunction that sets the CEL, it will also disable VSC and TRAC. And yes, these functions are truly disabled. The reason is that active stability control involves complex control of braking as well as engine power, and if the engine isn't running properly, the stability algorithms can't make the right decisions, so Toyota disables them.

2) If you only saw the lights last night, I don't think you can say much about fuel economy just yet. The mpg indicator on the dash is fairly poorly done. You'd need to run at least a tank through the vehicle and carefully calculate mileage to know for sure. I'd ignore that data point for now.

1) The O2 sensors could be at fault, but it would be somewhat remarkable for both banks (P0420 and P0430) to fail simultaneously. Same way with the cats. I would reset the codes for now and see what happens in a couple of days. Here is what the FSM lists for causes of P0420/0430.

- Gas leakage from exhaust system
- A/F sensor (Bank n, sensor 1)
- Heated O2 sensor (Bank n, sensor 2)
- Three-Way Cat converter

All of these are bank specific, so having both sides fail at once is hard to explain.

On the other hand, 135000 miles is about as long as the O2 sensors can be expected to last. I just replaced both of mine on bank 1 (driver side) at 150,000 miles. About $160 for both from Rock Auto (Denso's).

The rear one is a 10 minute job. The front one is more difficult. The best way I found is to remove the front wheel and the rubber splash shield and go in with a crow's foot wrench from the side.

Ron,

Thanks for the thorough reply.

This might sound dumb, but is there any hazard to driving around without those functioning?

Also, were those part numbers that I listed the same ones you purchased?

Unfortunately, I don't seem to have the order info around anymore for those sensors. Usually I keep the confirmation email, but I seem to have lost this one.

If you put the right specs for your vehicle into Rock Auto, you should get the right part. They're pretty good with that sort of thing.

As far as driving around w/ VSC and TRAC lights on, no problem mostly. The only time you might wish those functions were working would be if the roads were slick. Even then, depending on how old you are, you've probably spent most of your driving life w/o those functions, so just drive normally and you'll be fine.

The biggest potential issue is that, if the mixture isn't being controlled properly, it can eventually damage an otherwise good cat. That ups the repair expense significantly. If you reset the code and it comes back fairly quickly, I'd be inclined to replace the O2 sensors, since they're relatively inexpensive and "easy" to change, and they're probably due anyway.

BTW, I believe my Toyota dealer quoted me ~$400 to replace one bank of O2 sensors. Definitely worth doing yourself if you can.

I've had this issue pop up so many times on my RX350. Check the coil packs, not sure if they affect these engines, but I know that Camry / ES / RX has been plagued by the damn coil packs.

This happened to me recently and upon searching the forums, the first thing you should do is check your gas cap. Many found that making sure your gas cap is on tightly could be the cure. I know it sounds dumb, but it may work. Or you may have to buy a replacement cap if the seal is worn- a $15-20 purchase.

If that doesn't work (which it didn't for me), I got the codes read and one of my O2 sensors was dead. Got it replaced and bam, everything is running well again. I had an inkling that it wasn't the gas cap for me because my exhaust was smelling a bit rich, so when I found out an O2 sensor was dead, it made sense.