Emission warning light

[thegriffin]

The emissions light keeps coming on on my
Vectra 1.8 z18xe.
It has just passed the MoT so the emissions
are okay.
Have changed the downstream O2 sensor and
cleared the fault code but the lamp comes on
again the next morning.
The engine runs perfectly.I believe this lamp is
for info only and does not affect the engine in
any way ,so can I just remove the bulb assuming
it is not part of the MoT ?
I can't see any wiring fault.

[Robsey]

Have you got a fault code?

If the lamp illuminates for more that 5 or so seconds, then a fault code should be stored.

Hard to diagnose an exact issue without a code to narrow down the list of suspects.
O2 sensor, EGR valve, Cat converter etc.

The engine light possibly may not fail the MOT if it stays on.
It will fail if the MOT if it does not illuminate at all prior to start up.

The net effect of an emissions control error is that your Vectra will be more thirsty on fuel (potentially).
At worst, you could potentially kill the catalytic converter. -but quite unlikely.

[thegriffin]

The code is PO140 O2 no activity detected.
I can clear it but it won't extinguish the next day .
Are you sure about the MoT ?

[Robsey]

The official spiel about that code.

P0140 - OBD-II Trouble Code

Oxygen Sensor Circuit No Activity Detected (Bank 1, Sensor 2)

Fault Code Definition
The Catalyst Monitoring Oxygen Sensor evaluates the rear most Catalytic Converter's emissions-reducing capability. Code P0140 is set when there is no activity in the signal from the Rear Oxygen Sensor.

Symptoms

Check Engine Light will illuminate
In many cases, no abnormal symptoms may be noticedIn some cases, the vehicle will fail an Emissions Test
In many cases, the NOx emissions will be elevated

Common Problems That Trigger the P0140 Code

Defective Catalytic Converter
Faulty Rear Oxygen Sensor wiring or connections

Common Misdiagnoses

Oxygen Sensor is replaced when the problem is a defective Catalytic Converter
Oxygen Sensor is replaced when the problem is faulty sensor wiring or connections

My thoughts -

Did you have the sensors scoped to see the differences in output?

If you did, then we consider the following -

Both output signals (front and rear sensor) are the same or virtually the same = normally defective cat.

Rear signal is a flat line (no apparent signal) = defective wiring between the rear sensor and the ecu.

Rear signal has very small output variation - but not completely flat = defective rear sensor.

Remember - a new part can be defective... Especially if not an original factory spec (Bosch or similar) make.


I shall pull up official MOT rules to confirm.
Although if it has just got a new pass certificate, then this should not be a worry for you until next time.
But removing the bulb will hide any faults that may develop in future.

[Robsey]

Okay - just checked through most recent descriptions and guidance.

It appears that only safety system warnings are failable items.
So air bag or abs / esp warning lights.
but only if they indicate a fault.
if the light does not illuminate, then a fault cannot be assumed and therefore cannot be a source of rejection to issue a test pass.

Engine lights on or off do not give cause for rejection at the test.
Any reasons for rejection due to emissions problems will be picked up during the emissions test.

So in short - no engine light (CEL / MIL / EML) will not cause an MOT fail.

[thegriffin]

Many thanks for your advice it has been much appreciated.
I haven't got access to a scope unfortunately.
Apart from the wiring by the plug (which looks perfect)
I'm not sure where the most likely place to look next.
I don't really want to be cutting in to the Loom and
causing other problems.
Is there any way of testing the new sensor? It's a OEM
Delphi.

Cheers

[Robsey]

Another thought that comes to mind -
A bit random, but is your engine ecu connector(s) clean, dry and free from corrosion?
Not sure where the ecu is on your particular car, but they are usually somewhere where either moisture or heat can cause issues.

Regarding your sensor connector - you shouldn't need to cut into the wiring... just give any pins or sockets a good clean and dry if necessary.
Being behind the cat, the plug is close to exhaust so exposed to plenty of heat, road muck and moisture.

[thegriffin]

It's funny you should suggest that.I'm currently trying
to locate the ecu. It seems to be on the back of the
engine.
When I'm feeling brave I'll disconnect the battery and
pull the plug off and on a few times .
The wiring looks okay to it.

[thegriffin]

Have done that and am now getting code PO136
sensor circuit 2 . I'm not sure if that's an improvement
or not .

[thegriffin]

I have a spare 02 sensor for the PRE cat engine
management.
I'm tempted to plug it in and see what happens to the
fault code.
Could it do any harm to the ecu? It looks identical
except the lead is longer.

[Robsey]

It won't hurt the ecu.
Not 100% certain that it is the same sensor apart from the cable length, but you should be able to use the pre-cat sensor in the post cat position to at least see if it clears the code.
Assuming that the connector is the same.

Just be sure that you keep any excess wire away from hot or moving parts.

[thegriffin]

Plugged in known good sensor (but not screwed into
the exhaust ). Now have no fault or pending codes.
Replaced new sensor and codes return.
Assume have bought a faulty sensor and am now awaiting
a replacement from the supplier.
Will post again when I've fitted it .Fingers crossed.b

[Robsey]

Good to see that I wasn't spouting rubbish.

It can leave you doubting yourself when you replace faulty parts with new parts,
but the fault doesn't go away.

You sort of end up convincing yourself that a new part must work,
and so the fault must be elsewhere.

Looking forward to seeing a big fat smiley on your next post.

[thegriffin]

New sensor arrived as promised.I plugged it in and
this one doesn't work either !
The quality control on these gadgets is pathetic .
It seems that the heating element is faulty because
it does not get warm like my "spare "sensor.
I'm getting a little annoyed now.
Supplier is sending another for Friday.If this is faulty
I'll send them all back and fit my old pre cat sensor.
I'm afraid the smiley is on hold ufn .

[Robsey]

What a total pain...

I think that in your position, I would sack the replacement sensor and fit your spare pre-cat sensor instead.
At least you know it works.

As said before though -
Make sure that the lead is routed away from sources of heat or damage from moving / vibrating parts.

[thegriffin]

Have now fitted a 3rd new sensor which appears
to work.It's made by Auto Mega which I'm
not familiar with.But at least it works !

Hopefully that's the end of this episode .Only
time will tell ......

Big thanks again to Robsey

[Robsey]

My pleasure -

Good to have another Vauxhall happily doing what it is meant to.