Pre cat O2 sensor reading always 1.00. Topic is solved

[Markus123]

Hi, having issues with front O2 sensor readings. Its 1999 V70 b5254S engine with Denso managament.
Replaced O2 sensor (denso), did not help. Measured volts at sensor heater pins, they get voltage. Post cat sensor reads around 0.7-0.8V. Short fuel trim and front O2 reading always at 1.00, no movement at all.

What else there is to check, readings should fluctuate from 0 to 1 volts constantly?

[Markus123]

All O2 sensor readings at idle.

[454cid]

What are you using to read the sensor? When I look at O2 data, I graph it over time so I can see the change. I use the 850OBD2 app.

[Markus123]

What are you using to read the sensor? When I look at O2 data, I graph it over time so I can see the change. I use the 850OBD2 app.

Its chinese clone of Vida Dice.

[dikidera]

This is fine.

Anyway, 1.00 is not VOLTS, it's Lambda, the stoichometric fuel ratio, stoich is good. Anything above lambda 1.00 is considered LEAN. Anything below 1.00(such as 0.98) is considered rich, for a wide-band oxygen sensor.

All fuels require lambda 1.0 except in high RPM where liquid fuels help with cooling of the combustion chamber, giving a small knock protection. In all other instances, you want Lambda 1.0 for full combustion of the fuel mixture. Gaseous fuels such as LPG and CNG do not have this benefit and will instead choke an engine as they displace air and also contribute to higher heat.

Btw, Lambda is NOT air-to-fuel ratio which is 14.7(Lambda 1.0) for gasoline, 17.2(for CNG) etc.

Narrow-band oxygen sensors like the POST-cat sensor oscillate between rich and lean in a sine wave I believe.

When pressing the brakes from a high speed(as well as completely removing foot off accelerator in high speeds), your lambda should read 1.28(fuel cut off) and around 4500 rpm and above should be slightly rich around 0.95.

[Markus123]

Thanks for reply! Yep my bad, its lambda reading. Its strange as both pre cat and post cat sensors are 4 wired sensors, look idendical, so they must be narrow band? The current reading on front sensor -96.07 mA is strange also.

[dikidera]

They are nor equal.

The first oxygen sensor pre cat measures the exact air to fuel ratio so it can be 0.95 or 0.96 or 1.03 etc the exact, it's a wide band sensor. Post cat is a regular sensor measuring either rich or lean, it should be narrow band.

Additionally each of these sensors uses a reference air leeching via the insulation to produce the correct measurement which is why a pinched shroud or oily connector can cause a sensor to produce erroneous values. It's specifically mentioned in VIDA.

My information is for the later Denso 2.4 engines, but I am fairly certain it should still apply to you.