Rottenacura

Hey Everyone! I need some help here. I was driving home the other day and got the dreaded P0420 and P0420 codes. 170,000 miles on the car. The trim levels (long and short) seem a little lean (-3 or -4 on both sides). The front O2 sensors seem normal, however both rear (downstream) are fluctuating between .1 an .9 volts. Also, the scanner says the cat's are getting to 1500 - 1700 degrees.

Both out at the same time? The car makes good power. Idles great. But the economy fuel has dropped since the CEL. from 25 MPG to 18 mpg

I'm going to replace both rear sensors first. Denso or NTK?

Then buy new oem cats?

Could I have an exhaust leak? Too much air?

What else could I be missing here? I also, run the car out of gas 100 miles before this CEL came on. Can't see how that's related.

BROlando

The codes are just saying your cats are below efficiency.

They're reading more rich than a downstream sensor should read in closed loop. Meaning the cats are not working.

The car won't run any differently. But yes...the ECU *may* try to slightly lean out the mixture to try and fix it.

csmeance

have you cleared the codes and see if they came back? Have you tried any over the counter products like cat clean?

Rottenacura

So, I put in the o2 sensors (all four). Car does run better, but still have the codes. I bought some snake oil callled cata clean. That was a no go.

Looks like a need need cats. I did read that the codes could be triggered if only off by 5%. Is it worth it to try Laquer Thinner in the gas 10/1 ratio. Thoughts?

csmeance

what brand sensors?

Rottenacura

I bought denso, but they were not the oem. I returned the denso and bought the ntk (5 wire, front) .. sensor 2 on both banks are fluctuating voltage.i used a smoke machine in the exhaust and intake...no leaks. Tried a new maf too.

Bummer. the best price i could find both oem cats was $1500 (including tax, delivery and core.

Aside from oem, is there another brand or after market i should be looking at?

Thanks everyone!

Rottenacura

I bought denso, but they were not the oem. I returned the denso and bought the ntk (5 wire, front) .. sensor 2 on both banks are fluctuating voltage.i used a smoke machine in the exhaust and intake...no leaks. Tried a new maf too.

Bummer. the best price i could find both oem cats was $1500 (including tax, delivery and core.

Aside from oem, is there another brand or after market i should be looking at?

Thanks everyone!

BROlando

...as mentioned prior, your cats are the issue....so buying O2 sensors wouldn't fix it.

the cheapest fix for your issue is an O2 spacer for the secondary sensors (don't use this on your primary sensors!). Use angled O2 spacers...similar to what people use with test pipes or HFC's. They move the O2 out of the exhaust stream and fool the ECU into thinking the cats are fine. You won't pass a visual or exhaust gas analysis inspection with this, however. Not sure what your state's test standards are.

If that isn't a viable solution, then yes...buy a pair of OEM cats.

First...I would encourage you to find out why your cats failed, though.

Frankyy

check eBay & see if anyone is selling any cats that came off a low mile TL, I wouldn’t spend $1500 for new ones

Frankyy

Any update? I just noticed my car is running lean about -3 short & -4 on long but I have no check engine light I just checked it to see how it’s running
I only have 50k miles on 14 TL FWD

BROlando

How would his resolution help you? And if there's no CEL, your car is fine. What is the worry?