Help me understand fuel trims.

[Sequoiasoon]

This is in my 2017 Accord, 2.4L Earth Dreams GDI, 6MT. I had a P0420 CEL Catalyst system Efficiency Below Threshold Bank 1. I cleared it, it came back in under 200 miles, cleared and came back again in about 125 miles IIRC. I did the Cataclean and got the cat nice and hot by running at about 4k for 20 minutes on ride home.

My downstream O2 was usually about .7-.8V on highway cruise (cruise on) when I started looking but would drop to .2-.3V randomly. The wideband O2 I don't recall moving from 2.2V when I looked. CEL has stayed off for the last 1000 miles. Backing into driveway I heard a slight exhaust leak so went to shop and it needed a new flex pipe. Today I decided to clean the MAF and TB as it hadn't been done since I had it almost 5 years and 73k.

This is what I'm showing now at cruise (cruise on) and idle at stop light. What to the fuel trims and A/F ratio mean overall? Anything else I should be doing? As long as CEL stays off I guess all is well. It gets 35+ mpg on highway cruise, over 550 miles per tank. VRP running now to try and reduce some oil consumption (1.5-2qts per 5k) which I know is no good for the cat.

 

[ka9mnx]

Was the flex pipe before or after the cats?

 

[Marcozi]

"Fuel trims" are the corrections made by the engine computer in its effort to keep the air/fuel ration at the optimum level, and to compensate for vacuum leaks. There are 2 fuel trims (FT): Long term fuel trims (LTFT) and short term fuel trims (STFTs). LTFTs are the baseline FTs. That is, they are calculated over a period of time/miles to compensate for a permanent "issue" with the engine. STFTs are instantaneous compensations for short term and/or intermittent issues. The A/F ratio is just that: the ratio of the mass of the air and the mass of the fuel. Your readouts shows that the app is using the grams of air per second.

 

[SC Maintenance]

Your long term fuel trims look fine -2.3% at both load and idle.

+/- 5% is pretty much perfect.

Fuel trims are simply adding or taking away fuel from the ECU baseline mixture based on whatever set of conditions. You won't get a lean / rich code until 25%.

 

[Devo242]

Nothing to add on fuel trims, others explained it well. Just wanted to add your O2 sensor voltage wide range is 2.2V in both examples, the sensor itself or its wiring could be an issue. Had similar issue on my Accord years ago, changed with a new genuine Honda sensor and no further issues. Not to overlook that oil consumption issue, that is cause for concern for cat longevity.

 

[Sequoiasoon]

Was the flex pipe before or after the cats?

Flex pipe is after the only cat that is right off motor but before the downstream O2 sensor

View drivers side toward front of vehicle.

 

[Sequoiasoon]

Watching downstream is interesting, 6MT so every shift it goes to 0.00v since off the gas. Does the same in gear, engine braking to stoplights etc.

 

[Trav]

Any exhaust leaking now, the flange on the cat looks a bit sketchy. A leak in that area can cause rear o2 issues.

 

[iCurious]

If it really did had an exhaust leak, your B1S2 voltage would be low not high.

Your fuel trims are fine.

The real problem is oil consumption. It is what kills the catalytic converter. The PCM has been programmed when the catalytic converter reaches to a certain threshold, it triggers the check engine light.

 

[ka9mnx]

Flex pipe is after the only cat that is right off motor but before the downstream O2 sensor
View attachment 293825

View drivers side toward front of vehicle.
View attachment 293826

OK. Before you repaired the flex exhaust pipe, the leaking pipe would skew the downstream O2 sensor readings and function.