Is O2 Sensor Fault Code 100% Correct?

So I have established that 1992 NZ P10's have the same OBD-1 setup
as the USA G20's.

I unscrewed the ECU checked for fault codes, got back 5 slow 5 fast = 55 = no problems...

So, why am I getting less than 20 mpg. I filled up and my economy is
comparable to a V8 landrover.. something is wrong.

How does the ECU decide if the O2 sensor is bad?
Can the sensor be bad and the ECU not pick it up?
Can I check the real-time function of the O2 sensor?
What else could cause such a huger loss in economy?

Help would be appreciated.

CHEERS...

There are a whole bunch of parts that can't self-diagnose.

I don't know diddly about engines, and can't find the answer in my FSM. Problem is, there are many many things that can cause bad gas mileage.

Here's something I found elsewhere:

What should I do if my fuel economy is excessively low?
If your fuel economy is excessively low, you may have vehicle related problems. You may want to contact your dealer or a reputable repair facility to check your vehicle. The following are some of the diagnostic checks which a mechanic will typically conduct for poor mpg:
* Perform "On-Board Diagnostic System

* Check" for diagnostic trouble codes (sensors or actuators which may be malfunctioning)

* Check for dragging brakes

* Check transmission shift patterns for slipping, use of all gears, lock-up operation

* Check ignition timing

* Check owner's driving habits including A/C usage, hard accelerations, carrying heavy loads

* Check emission control system

* Check vacuum hoses for leaks, kinks, proper routing

* Check tire pressure

* Check fuel type, quality, and alcohol content

* Check fuel pressure and fuel system for leaks

* Check air cleaner element

* Check coolant level

* Check ignition system for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits, cracking or improperly connected ignition wires.

* Check for proper calibration of speedometer

* In extreme cases, there may be engine problems such as poor compression or faulty fuel injectors

Cheers for that.

I have checked most of the things on that list.

To have such a massive affect on economy, it must be something
like a O2 sensor.

As far as I understand
On full acceleration ECU runs open loop => no O2 sesnor
info needed.

But
During low-mid revs ECU runs closed loop => needs O2 sensor
to optimise economy.

Does the ECU always pick up a bad O2 sensor????

Later.

Honestly, I don't know. My car had a faulty o2 sensor about a month ago, and the light DID come on.

I followed the precedure from SE-R Faq, and checked my O2 sensor.

It seems to work fine.
I checked for fault codes, but also checked the lean/rich
function of the ECU/O2 Sensor using MODE-II.
At 2000 RPM the sensor switched an average of 9 times during
10 seconds, which apparently indicates an OK sensor.

So, now I am totally baffled???????

Think it has to be something like the tranny not locking properly,
or perhaps compression?

If the hydraulic lifters pump up too much does this affect the
compression, e.g. loss into exhast/intake; coz I think I am
running too viscous am OIL.
I had it serviced last time, and the guy reconned that 15w50
was the go, but from my reading apparently you are supposed to
use 10W30, or even 5W40.

Wrong oil is what causes the cold stalling in these cars, just wondering if
the pumped up lifters, also inhibity economy?


Later

If you get the O2 sensor fixed you will see a great difference in your car, it will run way better.

Well....... i didn't get an O2 code. But my ECU was also short circuited.....

-Phil

Hey Domiken

As stated in my post, I have tested the O2 using the ECU and the system switches from lean to rich 9 times per 10 secs @ 200rpm.
This is better than the WSM suggests.
Are you saying it should switch more than this?
Do you think the O2 sensor is hosed?

Later

My o2 was stuffed and i got no codes on the ECU. The car was going like crap so i pulled it out and sprayed it with carb cleaner and let it drive. Over the next 30 mins the car changed for the better so i brought a new one and haven't looked back.

My 2c

Hey Primal SMX

Did you do a Mode II diagnostic on the O2 sensor?
e.g. put the ecu into diagnostic mode, then start the car, and
watch the engine light flick on and off as the ECU goes from lean to
rich.

I checked my ECU and got no fault codes, so then did a mode II
check and found that the O2 sensor was working.
Problem is I'm not sure how well it is working...
I'm pretty sure O2 sensors dont just overnight decide not to work, but
get progressively less accurate.

You have a '97 so I cannot understand how your O2 was hosed, how
many K's have you done?
I have done 130,000k's so the O2 is probably due for replacement, but
dont want to change it unless it will cure my poor fuel economy.
If it is not the O2 sensor, I suspect it may be a clogged CAT CONV.

Later

Whats Self Diagnostics OBD II ? How do i turn it on?

I used Mode II ...... worked fine supposedly.
Oh btw..... i replaced my ECU (after getting 20mpg)...... runs 25mpg.

Check engine light is STILL ON!

HEEEEEELP :)

Anyone know where to get the ECU light codes? And how to turn on OBDII Diagnostics?

-Phil

Well most of my self-diagnoses are listed here. (http://www.automotiveforums.com/vbulletin/t49803.html)

Any chance the O2 sensor could be atleast part of the problem? The code didn't come up in mode II, but the sensor doesn't look particularly healthy at this point... :( I have no where left to go at this point IMHO :(

OK, so I haven't posted for a while...

But I did a mode II test again last night from cold.
Wet for a sedate drive, ECU stayed in open loop until
engine cam up to normal temp.
Then engine light went nuts ( lean/rich/lean/rich/lean.....)
so the O2 works fine. I might spray it with carb cleaner
when I finally get around to changing the oil.

I now think the problem could be the MAF, I checked the
earth potential difference between the middle pin
and a ground on the block... It had 17mV of difference...
too much, so I am going to put an earth strap on it.

Im confused though...
If the reference(ground) to the sensor is too high, then the output
voltage will read high.
Thus the ECU will think there is more airflow to the engine than
in reality. Therefore the ECU puts in more fuel than required, and
stuffs my fuel economy..

But, surely this is where the O2 sensor kicks in and leans out the
mixture. Perhaps the MAF has priority.
Not sure... Any ECU programming GURUs out there???

Anyone confirm my suspicions?
I will update the results of re-grounded MAF.

LATER