Technical Check engine light.

[John Connor78]

Hi everyone, first time here……I’m pretty much at the end of my patience. I have a automatic fiat 500 sport 2014. It had a new engine put in 6mths ago and ever since then the engine light remains on and once the car is started up tells me to check engine. My mechanic said at the time it was the 02 sensor may have been damaged when the original engine went (I didn’t think so) we replaced it. Things where fine then days later back on again. My mechanic reset the computer it went away then days later again it came back. I’ve been dealing with this for 6mths, I had an auto electrical mechanic scan the car and this time it’s claiming it’s the catalytic converter. I don’t know if it is, I have a feeling it’s the new engine that was put in as the 02 sensor didn’t originally change anything I doubt this other sensor will. Any ideas what you think??? Also, is this doing or going to do more damage to the car as time goes on me driving it? Thanks

 

[Ralf S.]

The light means that the ECU is getting a signal from somewhere that's "out of range". It could be something was previously damaged, or the engine work could have disturbed something in one of the electrical connections, or at the outside, the new motor and/or any sensors attached to it are slightly incompatible with the rest of the car.

How did your old engine die? If it suffered a catastrophic failure and it dumped a load of oil through the exhaust (e.g. if it dropped a valve and punctured a piston.. loads of smoke out of the back) then the lambda sensor(s) and Cat' could have been contaminated. If the engine just seized or broke a rod etc. then the exhaust system would be oblivious to all that, and a contaminated Cat/lambda would be very unlikely.

What actual code is the ECU reporting? I bet someone on here had it and fixed it.

I don't know how the ECU would know the Cat' is damaged.. all it's got to go on is the lambda sensor. Your car may have a pre-Cat and also a post-Cat lambda(s)... so the post-cat sensor might be reporting something odd. Give it an inspection. But if you have just one sensor, it will be fitted pre-Cat.. so there's nothing to tell the ECU what's happening downstream of that. Your Cat is likely okay.. even unburnt oil tends to not kill them.

Check that the lambda *connector* is immaculate, clean and tight. It may be worth fitting another new one, but buy an NTK or Bosch doodah, not a spliced-in cheapo' one. Lambdao's things work on resistance variances of less than half an Amp or something... so you need one with an integrated plug fitted, not one that you have to cut and splice-in to your original plug.

It won't be your engine. That starts and runs okay, apparently... so it's fine. You won't do any damage to it driving with the light on. At worst, it will use more fuel and be down on power because the ECU doesn't know what the lambda (or something) is telling it, so goes "cautious" (i.e. rich).. since this doesn't damage the engine. How is your fuel economy/power? I guess "no difference"?

If you accelerate slooooowly in the same gear from 1000rpm to 2000rpm.. have a "seat of your pants" feel for what the engine is doing. Do you get a hesitation around the 1500-1800 rpm mark? The lambda includes a heater to keep it warm at low revs, and that turns off when the engine is using more revs. They turn off around 1600rpm ... so if your lambda is a bit weedy, you'll notice a hesitation when the heater cuts out and leaves the sensor to fend for itself.


Ralf S.

 

[Goudrons]

There are two O2 sensors on these (and on most single banked engines), one before the Catalytic Converter often called the Pre or Upstream sensor and one after it, often call the Post or Downstream sensor.

You can see the the Pre sensor under the bonnet sticking out of the exhaust manifold heat shield if fron of the air box.
The Post Cat sensor is seen from under the front of the car, screwed into the right hand side of the lower part of the manifold/Cat.

The Pre Cat sensor controls the fueling, it basically self generates a voltage depending on heat from the exhaust gases.
Cool is rich and generates a low voltage, so the ECU takes fuel away for the engine.
Hot is lean and generates a higher voltage, so the ECU adds fuel to the engine.

The voltage is anywhere from 0.1v to 0.9v and perfect long term, mid or average "trim" should be 0.45v.
In reality the ECU gets a wave of data back, low to high, high to low and so on.
On idle this "switch" in voltage will happen around once every second, but as the revs climb it will speed up.

It's said the Post or Downstream O2 sensor does nothing except monitor the Cats condition.
But what it's actually doing sending another set of voltage figures so the ECU can compare what's going on.
The ECU is expecting a different set of "switch" data from this sensor as the Cat should have altered the pre "switch" data it's seeing.

Quite what this altered reading should be is questionable as the Cat works better the hotter it is, so it's setting for it to suddenely state "I have a problem here" is fairly rare, but when it does it can also start to influence the fuel injected (a safe run rich emergency setting ) as it tries to protect the engine from getting damaged.

It's difficult to tell without the actual code but it does sound like there is a problem with this Post Cat O2 sensor or what it it reading.
There are two different things.
One is the sensor or wiring to it is faulty which can cause an open/closed circuit type fault code or an under or over reading type fault code.

The other is there is an actual Cat problem or a fueling problem (as excess or insufficent fuel injected will obvious effect what comes out of the Cat, obviously you wouldn't get the open/closed circuit fault but an over under reading type of fault.

It's a shame you haven't the actual code so we could work out the best way of tackling the problem.
As a start you might want to get the Post Cat sensors wiring checked or even replace the sensor.

 

[John Connor78]

The light means that the ECU is getting a signal from somewhere that's "out of range". It could be something was previously damaged, or the engine work could have disturbed something in one of the electrical connections, or at the outside, the new motor and/or any sensors attached to it are slightly incompatible with the rest of the car.

How did your old engine die? If it suffered a catastrophic failure and it dumped a load of oil through the exhaust (e.g. if it dropped a valve and punctured a piston.. loads of smoke out of the back) then the lambda sensor(s) and Cat' could have been contaminated. If the engine just seized or broke a rod etc. then the exhaust system would be oblivious to all that, and a contaminated Cat/lambda would be very unlikely.

What actual code is the ECU reporting? I bet someone on here had it and fixed it.

I don't know how the ECU would know the Cat' is damaged.. all it's got to go on is the lambda sensor. Your car may have a pre-Cat and also a post-Cat lambda(s)... so the post-cat sensor might be reporting something odd. Give it an inspection. But if you have just one sensor, it will be fitted pre-Cat.. so there's nothing to tell the ECU what's happening downstream of that. Your Cat is likely okay.. even unburnt oil tends to not kill them.

Check that the lambda *connector* is immaculate, clean and tight. It may be worth fitting another new one, but buy an NTK or Bosch doodah, not a spliced-in cheapo' one. Lambdao's things work on resistance variances of less than half an Amp or something... so you need one with an integrated plug fitted, not one that you have to cut and splice-in to your original plug.

It won't be your engine. That starts and runs okay, apparently... so it's fine. You won't do any damage to it driving with the light on. At worst, it will use more fuel and be down on power because the ECU doesn't know what the lambda (or something) is telling it, so goes "cautious" (i.e. rich).. since this doesn't damage the engine. How is your fuel economy/power? I guess "no difference"?

If you accelerate slooooowly in the same gear from 1000rpm to 2000rpm.. have a "seat of your pants" feel for what the engine is doing. Do you get a hesitation around the 1500-1800 rpm mark? The lambda includes a heater to keep it warm at low revs, and that turns off when the engine is using more revs. They turn off around 1600rpm ... so if your lambda is a bit weedy, you'll notice a hesitation when the heater cuts out and leaves the sensor to fend for itself.


Ralf S.

Ralf s. Thanks so much for the information I appreciate it mate.

 

[John Connor78]

Thanks guys the code is P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD BANK 1

I’m taking it to another mechanic as the initial mechanic who did the replacement engine install is a muppet and unless I can prove it was his mistake with the install or the replacement engine I have no case. I just can’t believe it was from the initial engine bombing as this has been ongoing since October.

 

[A3jeroen]

Don't know which O2sensor was replaced, but could this be a problem with the post cat sensor?

gr J