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(Post Link) post #1 of 8 Old 24-08-11 Thread Starter
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Engine warning light

Hi, this is my first post so go easy on me, i really just want to make sure my logic is sound on this one.

I have a 2004 156 2.0 JTS.

A little while ago the engine management light came on and as i dont have a fault code reader (am going to buy one at some point) i called the AA out to get the code out for me.
They got the code as P1151-Lack of HO2S-21 Switch, Sensor Indicates Lean

So i took it to an Alfa/Fiat specialist who said it was most likely one of the lambda sensors which was what i thought, so they replaced one, and cleared the code and said if it comes on again they will keep the original sensor and swap it round with the other one.
It came on again again about 8/9days later so they swapped it over.
After about 2 weeks the light came on again, i havnt checked to see its the same code yet but i assume it is.

A little while ago i noticed the engine takes a little longer to warm up than most cars i've had. So it got me thinking, i wonder if a faulty thermostat was actually the cause of this, the mixture would possibly be off due to this when initiall warming up. Does this seem plausable?
I will also say that each time its come on its been on a short-ish journey about 3/4 mins into driving it (i.e when i'd expect the engine to be warming up).

Should i go ahead and replace the thermostat to see or am i missing something?

Sorry its a bit long but best to have the full picture eh.

Cheers, John
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(Post Link) post #2 of 8 Old 25-08-11 Thread Starter
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nobody have any idea?
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Hi John

I have a 2003 156 JTS 2.0 and the same thing happened to me.
The time frames were a shorter than yours but i've had the lamba sensor changed after the engine warning light came on. Then it came on again and it turned out to be a faulty thermostat. Had the thermostat changed last Thursday and the car has been fine since.
(Post Link) post #4 of 8 Old 29-08-11 Thread Starter
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thanks mate, i thought it might be the case.
will get this changed.

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Unfortunately not just ALFA but many marques take their ECU readings from the temperature sender unit.

Faulty thermostats can cause many, many faults and code reading only gives half an answer and never the initial problem.
Bad design to have the temperature sender unit housed along with the thermostat in the first place.
At least you know the O2 sensors are changed and should last now so it's not all bad mate.

A lesson for us all there.
Check codes by all means, but never trust them as gospel. This day and age sensors are fairly long lasting and reliable, yet ODB codes always lay the blame on the sensor not the root cause simply because they can't. Use of old school mechanical logic on parts along with codes and you have a sound combination
(Post Link) post #6 of 8 Old 08-09-11 Thread Starter
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well, the new thermostat is in and i've cleared the code so its just a waiting game now i guess. will come back in a couple of weeks and let y'all know if this has worked. engine is running much better now, straight up to temp and stays planted there, should make demisting the screen this winter easier and now its running properly i should be using a little less fuel too.
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(Post Link) post #7 of 8 Old 12-09-11 Thread Starter
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well, this didnt work, MCSF light came on again a few days after, i am starting to run out of options/ideas,
have replaced both o2 sensors, checked wiring thoroughly as has the AA recovery guy, replaced thermostat, had engine diagnostics done to see if it genuinely was running lean.

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Member car:

Not an Alfa

Is it still P1151?

"A code P1151 may mean that one or more of the following has happened:


Short to VPWR in the harness or HO2S
Water in the harness connector
Open/shorted HO2S circuit
Corrosion or poor mating terminals and wiring
Damaged HO2S
Damaged PCM

Fuel System:

Excessive fuel pressure
Leaking/contaminated fuel injectors
Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system

Induction System:

Air leaks after the MAF
Vacuum Leaks

PCV system:

Improperly seated engine oil dipstick

EGR System:

Leaking gasket
Stuck EGR valve
Leaking diaphragm or EVR

Base Engine:

Oil overfill
Cam Timing
Cylinder compression
Exhaust leaks before or near the HO2S(s)"

If you can play a guitar and harmonica at the same time, like Bob Dylan or Neil Young, you're a genius. But make that extra bit of effort and strap some cymbals to your knees, suddenly people want to get the hell away from you.
- Rich Hall

Gone but not forgotten - 164 2.0, 164 Super 3.0 V6, 156 1.6 T.Spark, 156 2.5 V6...

Last edited by Ronald; 12-09-11 at 18:53.
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