2006 Honda Odyssey EX 3.5 - P0420

Working on a 2006 Odyssey EX with the J35 (no VCM.) 160K on the clock and it has a P0420. That's the easy part. The harder part is the "why" it has a P0420. About 25K ago I put four O2 sensors in this car which solved a bit of what the customer called "chugging." When I drove it back then I felt an ever so slight hesitation but nothing significant. This was back in March, when it had 132K. At that time I noticed the fuel trims a little high, i.e. 9-10% LT and 10-15% ST) but no lean codes. I checked everything and could find nothing definitive so I decided to replace all four O2's with NTK (OEM supplier) which solved the "chugging" problem and it's been running smooth for her ever since. After I replaced the O2's I was sure the fuel trims would come down. While they did not improve, the slight hesitation and "chugging" was gone so I considered it fixed and customer was happy.

Fast forward to today. She brings it in with the MIL on. Scan for codes and it has P0420. So, it needs a cat but I'm concerned that the elevated fuel trims, or whatever is causing them, may have contributed to the P0420 code. I don't want to replace the cat without getting to the root cause of the elevated fuel trims. As of today, the fuel trims at idle are at 15% ST and 8-9% LT. It sets no lean codes.

The ST fuel trims do improve just slightly at 3000 RPM which made me suspect possible vacuum leak. I smoked the intake and everything appears tight with the sight gauge on the smoke machine showing about a 0.025" leak and no smoke anywhere except just a trace at the EGR which is pretty normal in my experience. MAF at idle (around 675rpm) was showing about 2.9-3.0 g/s and MAP voltage at about 0.78v. I cleaned the MAF with no improvement. I pinched off the brake booster and purge lines with no change either.

What am I missing? I can't confirm that this engine has ever had a valve adjust, and at 160K it's quite overdue. I don't think it's ever had a throttle body cleaning either. Would either of these things elevate the fuel trims? I didn't do a WOT test since I didn't expect a fuel delivery issue since the numbers improve slightly with RPM.

Thanks for any ideas on this one. Every time I've had this happen on a J35 with A/F sensors I have fixed it with a full set of O2's. This is the first one that hasn't been fixed with O2's. I'm leaning toward selling a new cat along with a valve adjust, pcv, and TB cleaning but want to get some input on anything I might be missing. At 160K it needs a valve adjustment anyway. For now I reset the PCM and sent her on her way to see how long it goes until it comes on again.

Thanks for any ideas,
John

I can't see that you're missing anything, sir. Your plan of attack sounds good to me! The only other thing I'd suggest is plugs, if they haven't been done recently.

Your MAP reading suggests you won't find any tight valves. More often than not, I find them too loose and end up tightening them to spec. I don't think a dirty throttle would cause a trim issue, but definitely give it a cleaning anyway. Any chance of ethanol fuel? That'd definitely explain the lean with no faults found.

Bank one is the firewall side, right? If you're gonna have that axle out for the cat, you might also consider resealing the crankshaft main bearing cap bolt while you're there. There's a TSB for an oil leak from that bolt - just remove the bolt, clean everything up and use RTV.

You might consider an injector service....If the TB never was cleaned nor the valves adjusted then the injectors might never have been cleaned neither.

Does the bank 1cat fail completely? How is the B1S2 O2 signal and what does a comparison between B1S2 an B2S2 sensor reveal?

juergen.scholl wrote: You might consider an injector service....If the TB never was cleaned nor the valves adjusted then the injectors might never have been cleaned neither.

Does the bank 1cat fail completely? How is the B1S2 O2 signal and what does a comparison between B1S2 an B2S2 sensor reveal?

What do you mean by "fail completely?" It either fails the OBD test or it doesn't. I could attempt to look at mode 6 data but my Autel DS708 doesn't decypher that data and decoding it can be time consuming. I did manage to decode some on a Toyota Sienna but the service manual gave some of the mode 6 data info.

I didn't look at the two downstream sensors and I probably should have. With A/F ratio sensors in front, though, it can be more difficult to determine what the computer is seeing that it doesn't like but I will look when I get the van back.

As for an injector cleaning, I suppose that's an option but I've never done it. I occasionally run a bottle of Lucas fuel treatment in my own Ody tank but from everything I've read I'm probably just wasting my money. Where she buys her fuel, and what type, might be a good question, though, and I will ask.

The catalytic converter may underperfrom and fail the onboard test. This well may be due to other reasons than the catalytic converter itself, especially if there is a problem with the fuel injection system present (garbage in - garbage out). If you had an gas analyzer - which does not seem to be the case - it would be easy to check the function of the converter.
Instead you can check the oxygen storage capacity of each converter with your autel scanner in graphing mode to get an idea of its state of health...

The fuel additive indeed is a complete waste of money in your case. I am very sceptical of its usefulness in general. Furthermore it is thought of as preventive maintenance, not as corrective aid.

The proper way is to clean the injectors in a laboratory with an ultrasonic bath. You'll be able to see the before and after of the spray pattern an flow and the outcome maay well surprise you.