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AF/O2 and Fuel Trim data - Normal or not?

I’m learning to diagnose my car’s poor gas mileage:

Car: 2010 Toyota Matrix (Base). Engine: 1.8L, 4 cylinders, Auto, FWD. ODO: 103,000km (64,000 miles). City gas mileage: 12.6L/100km = 18.6 mpg (advertised City gas mileage is 8.1L/100km = 29 mpg)

Photo attached is data for AF, O2 sensors and Fuel Trims at idle (please click on the photo to expand the full data).

Please advise me: (1) if everything is normal or not? (2) if any other data should be read?


Your O2 sensor looks a bit lazy. I.e. changes a bit slow. Data shows a lean mixture on over-run every time you crack the throttle. That’s normal.

How is the long term fuel trim? Positive? Negative? Short term fuel trim? Does it switch quickly?

Hows the coolant temp? Does it come up to temp quickly? Bad thermostat can give poor mileage.

Any error codes at all? Does it run OK in general? Just bad mpg’s?

Advertised city fuel economy is 25-26 MPG (US gallon)

18.6 MPG is low either way though. I agree with @Mustangman The O2 sensor readings seem slow, particularly on the 2nd and last jabs of the throttle. I don’t think that alone is the problem though…

There’s difference between a lamda O2 sensor signal,

And a wide band O2 sensor signal.

Looks normal.


Thanks @Mustangman.

Your O2 sensor looks a bit lazy

Do u mean the upstream AF ratio or the downstream O2 sensor? Is downstream sensor supposed to be steady instead of fluctuate?

How is the long term fuel trim? Positive? Negative? Short term fuel trim? Does it switch quickly?

Long and Short FT are included in the photo (please click on it to get it shown fully). I’m not sure if their readings are good or bad. Please give me some advise.

Any error codes at all? Does it run OK in general? Just bad mpg’s?

No code at all. Run fine. Just poor gas mileage.

Hows the coolant temp? Does it come up to temp quickly? Bad thermostat can give poor mileage.

I’ll check coolant temp and post shortly. What is the best way to test this? At idle then increase rpm, or drive?

Do u think 18.6mpg is bad city gas mileage (I drive quite gently, not aggressive at all)? What other data should I read to diagnose this?

Does this describe you?


  • Short trips: true (4-5 km a day plus sometimes 15-20km weekends or in the middle of the week)
  • Cold weather: I live in Toronto. The gas mileages were measure from September to October (900km) - not very cold and no snow yet.
  • Sometimes in morning warm up the engine and turn on heater and fan inside for 2 mins before driving.
  • Tire pressure around 32 psi. Sometimes 30.5 - 31 psi.

I used to drive a Camry 2.4L in my home country, with much much much worse traffic but still max 10.5 - 11 L / 100 km

Limited experience w/diagnosis of those sensor signals, but to me it appears the o2 sensor before the cat (sensor 1) is not working correctly. It isn’t going up and down every few seconds like I’d expect to see. For example like the signal in the first diagram in Tester’s post above, under “normal operation”. This presumes it is a standard binary zirconium lambda o2 sensor. If it is a wideband sensor then the signal may be ok.

The other signals look normal, except for the long term fuel trim, which is biased to about 5.5% negative as I recall. That’s within normal range, but if any of those signals is a clue to your mpg problem, that’s the one I’d focus on. (That could be caused by the unexpected o2 sensor signal I noted above.)

There’s a second long term fuel trim labeled “total” which is biased at nearly 0%, which is good, but I don’t understand how the long term could be negative 5.5% and the “total fuel trim” 0%. Does your manual have an explanation for the difference in long term and total fuel trim displays?

As far as your decreased mpg, the most common reason sans testing evidence is usually the engine is running under the designed operating temperature, due to a faulty (partially stuck open) thermostat. Something like that wouldn’t be unusual at all in an 8 year old vehicle. Thermostats tend to warp a little as they age and don’t seal 100%.

With those short trips and cold idling 18 MPG is a reasonable achievement. With some effort you may do better and it doesn’t involve repairing the car.

I think you should take a longer drive on a weekend at a reasonable speed and see how close you get to what the vehicle is rated at. Cold weather and short trips are not what gives decent mileage.

Yeah, my sensor 1 is wide band. The (interpreted) voltage reads around 3V.

I’m not sure about this. But that might be the sum up of Short and Long?

Good point.

My guess is the total fuel trim is the ecm corrected long term fuel trim. In other words the long term fuel trim is telling the ecm it needs to inject less fuel than the maf (and other non-o2 sensors) is telling it, so it applies that correction to the amount of fuel injected, and the result at the tailpipe is the total fuel trim. 0% means the ecm is making the correction accurately and achieving the desired a/f mixture. But that doesn’t explain why it needs to make the correction in the first place. A negative long fuel trim means w/out correction there’s either too much gas or too little air getting into the engine, and could indicate a fuel pressure problem, clogged air filter, exhaust restriction, etc.

Bottom line though, other than verifying the coolant temperature is correct, this isn’t something I’d worry about myself. At least not until I took a couple hundred mile drive on the freeways and measured the mpg for that trip. Or do what I do w/my Corolla, never measure the mpg, as long as the engine is running smoothly, why worry!! … lol …

On a side-note, if your engine requires periodic valve clearance measurements, that could be related.

The engine should reach operating temperature of 82 C by about 5 miles of travel, if it takes more than approximately 8 miles to reach operating temperature the check engine light should turn on.