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[demi 900]

Hi all, had a p0125 fault...lambda sensor come up. Is their any way of knowing whether its the pre cat sensor or exhaust sensor?

[gregk2]

Are you sure it's lambda sensor?

Common Problems That Trigger the P0125 Code

Defective Engine Thermostat
Defective Engine Coolant Temperature Sensor
Defective Intake Air Temperature Sensor
Defective Cooling System
Low Engine Coolant
Dirty Engine Coolant, causing incorrect Coolant Temperature Sensor readings
Defective, always running Engine Cooling fan(s)

Common Misdiagnoses

Engine Cooling Fan
Internal Engine problem
Oxygen Sensor problem
Engine Coolant Temperature Sensor problem

http://repairpal.com/OBD-II-Code-P0125

[demi 900]

yes i am sure, thats the result on a star machine. But does not state which sensor needs replacing!!

[demi 900]

Also is their a specific make or part number the o2 sensors?

[Alex]

The star would surely point out which sensor it is, even a cheap ODB2 adapter + Torque signals if it's the upstream or downstream sensor, and if it's a problem with the sensor itself or just the heating element.

Who interpreted the fault codes when you diagnosed it?

[demi 900]

The guy who owns the garage. I watched him do it. It just said o2 lambda sensor. He turned the light off. I drove 3 miles home. Next morning 4 miles down the road, the light came on again.

[Problemchild]

i've just started having this as well.... i think the higher ambient temp has been causing it tbh

you have the same set up as me??

OEM turbo but non OEM manifold plus sports CAT?

get an OBD2 dongle and an app for your phone and just clear the fault

mine has come on 4 or 5 times in the last few months

JJ

[Alex]

If you can see no noticeable drop in power and fuel economy then it's the downstream lambda sensor.

The heating element of the downstream lambda sensor is a very common failure, and the light coming on means that it takes longer than specified for the sensor to get up to temperature, not that it doesn't work. The ECU just ignores lambda feedback and runs in open loop until the sensors are hot.

Try a different  garage or just get a cheap obd2bt adapter and torque. This way you can better monitor engine faults, and you can figure out why they happen.

[demi 900]

The light comes back on a few miles later JJ. Alex, is that the pre cat sensor you are talking about? The only thing I notice is the smell of fuel. So maybe running a bit rich?

[gregk2]

I had the same error code after de-catting.

[Alex]

Upstream is pre cat, downstream is after the cat, and is used for diagnosis, not to regulate AFR so there's usually no apparent running issue when it's heating element is dead.

If your's is running rich then it should be the upstream sensor if it's a lambda sensor problem.

[demi 900]

I am  going to put the cat back on, just to see if the light stays off. Failing that, i will buy an upstream sensor. Thanks for all your input.

[Problemchild]

if you have not made any changes to the car i would just assume its something to do with the ambient temp making the car run a little differently and the sensor saying its outside of parameters

if my engine light is on and the car is ticking over, when i clear the code just the engine note changes ever so slightly but no change in performance

it then stays off until the next hot day

sometimes it takes a few hours to come on

JJ

[demi 900]

JJ, i have had a few changes to my car... fiNe performance manifold smoothed and fettled, downpipe smoothed and flowed by fast eddie. Then i took it straight to tony for a 2nd re map. A week or so later the light came on. But it still runs fine, apart from the occasional rich fuel smell, and occasional hunting. I am going to bite the bullet and buy a new pre cat o2 sensor. By any chance, does anyone know the part number for this?

[Alex]

Is there no other place you can take it in to pin point the problem? Throwing parts at it might not be the most economical way to fix this, being modified.