Hey guys!
2006 Accord EX Coupe auto with the 2.4L 4-banger. 118k miles.

I replaced: exhaust manifold to engine gasket; front flex pipe, catalytic converter, and all associated gaskets; and finally the downstream O2 sensor.
I still have a p0420 code. What gives? Help!

 

Did you get any other codes? Did the P0420 immediately return after replacing all of those parts?

 

Originally there were no other codes. Upon hooking up my scanner today, there was no CEL, but a stored p0420. I cleared it, then changed the MAF (mounted on intake tube), a job the shop had advised me to also do. I then got a p0113, cleared that.
Let car cool down, took it on a nearly 1 hour drive…CEL came on with the p0420 again. This was earlier tonight..

 

Does your scanner provide any live data from the engine computer?

 

yes, it does have that function. Plus monitoring.

 

When the engine is warmed up and at operating temperature, monitor the short and long term fuel trims at idle, then at ~2000 rpm. Also monitor the secondary O2 sensor (bank 1 sensor 2) along with the map and IAT sensors.

Report the values at idle and at the 2000 rpm. They will jump around a bit, so an approximate value will work for a first pass.

 

awesome.. thanks so much. I will try and do that this eve if my son doesn’t take out the car. If so, then I’ll report back tomorrow morning sometime.

 

hey, good morning. Just wanted to check back in. It’s been a crazy few days.. I hope to get the car scanned tonight after work.. I’m very curious what the values will be. I hope I will have a comprehensive reply for you shortly!

 

Alright! So I just did the live data test. Here’s what’s going on: ***There was no CEL, and no stored or pending codes. I/M readiness is all OK except Evap monitoring (INC). NOW, the CEL came back on when I started the car up to park elsewhere. This time, it’s P0171- System too lean bank 1. I DID already just change the MAF. You can’t make this stuff up 😆

 

The scanner information is helpful, but missing a few items. Can you repeat the same test and include the MAF (not map) sensor along with the engine coolant temperature sensor(s) and open/closed loop. it may be called fuel status, or just google to find out how your scanner identifies open/closed loop.

Can you plot both O2 sensors at idle and 2000 rpm? From your screenshot the label would be O2S1 1 and O2S2 1?

Based off of this test, your car's PCM is adding fuel with the high short and long term fuel trims. The trims didn't change at higher rpm, so a vacuum leak is less likely. With the information so far, the MAF sensor can be way of, low fuel pressure, an issue with one or both O2 sensors, engine coolant temp sensor, or maybe the catalytic converter. The next step is to narrow down with further testing.

Do you still have the original MAF sensor? Do you have a propane torch? The propane torch would be used to add fuel and see how the car responds. We can explain later.

Does the car drive normally, or do you notice any loss of power when driving?