Rear oxygen sensor reading low

billmunny

Peugeot 307 1.4 3ac 2002 ( 8 valve).......just barely passed NCT with Lambda value of 1.02. The limits are 0.97 to 1.03.
I checked the 1st oxygen sensor(4w) on a scope and it shows oscillating between 0.2v and 0.85/0.9v at approx 1hz.
I cannot actually see a sinewave but the trace is showing these values. The engine is run at approx 2500 rpm during this scope scan.
I then tested the the 2nd oxygen(4w) sensor but I put a dvom on the two wires first and started from a cold engine ( 30 mins after 1st o2 test) and the initial value was 0.45 steady and then it gradually dropped until it stabilized at approx 0.125/0.150 volts when fuly warm with the fan kicking in once.....ie it seems to be reading low irrespective of the rpm.
The NCT result indicates its running lean and a low O2 reading is indicative of a lean mixture coming from the CAT .
Is this indicative of a failing cat?
There are no driveability issues or starting issues or engine issues at all.

whizbang

You really need to see what the ECU thinks 0.125volts means. This means reading the ODB data, unless you can find a listing of voltages somewhere.

But the diagnosis does make sense; If the sensor is falsely indicating a low O2 , the ECU should run a lean mixture to increase the O2, and this gives the high Lambda.

There may be other reasons. If the Airflow sensor is reading low, then the ecu will lean out the mixture too.

enumbers

I wouldnt have any concerns over 1.02 reading on a car that age a very slight exhaust leak could cause the reading. The 2nd sensor readings would appear to be a cat working well and as you shown your front sensor is swithing nice and quickly so fueling must be pretty spot on

billmunny

Hello
I figure it should read approx 450mV if it was reading a lambda of 1.0
I might try to remove this sensor and test it somehow and see for certain if it works okay. I will update with the results when done.
rgds

PukkaStukka

The rear O2 sensor is used to monitor the performance of the catalytic converter. It is not used to control the running of the engine.

You should see oscillation of the front sensor both at low speed and constant higher RPM. Your description suggests it is switching but unless you are using a scope with some form of trace storage then you will have a your work cut out interpreting the exacts of what you are seeing. You're probably better off reading this using a scan tool.

Once the car is warmed up and is running closed loop, the rear sensor should be running a flat line. If the car is cold, not running closed loop, or has a defective cat, or a running fault like a hefty misfire, you will see oscillation like sensor one and this eventually will put the MIL on and log something like a P0420.

Finally, I wouldn't worry about your lambda reading. It has nothing to do with the lambda sensors The fact that it isn't bang in 1.00 probably has more to do with the age of the car and internal wear inside the engine. New plugs and filters might help

billmunny

Thank you whizbang, enumbers and PukkaStukka for the replies.

I did change the air filter and sparkplugs before the NCT .

Where should the rear O2 sensor voltage flatline at ? ( assuming no issues)

PukkaStukka

billmunny wrote: »

Thank you whizbang, enumbers and PukkaStukka for the replies.

I did change the air filter and sparkplugs before the NCT .

Where should the rear O2 sensor voltage flatline at ? ( assuming no issues)

Somewhere around 0.4 - 0.5V