Really weird problem!

kowsmou said:
Ok so I have a 57 plate Z4 2.0 with 40k on the clock. I've been getting intermittent P0420 error codes last few weeks which is related to catalytic efficiency. When I say intermittent I mean I could go 300 miles and it won't come on! I took it to a BMW specialist who couldn't definitely narrow down what was causing it but he's going to change the pre-cat O2 sensor next week as it's covered by the warranty I have so it's not costing me anything. I have tried using Cataclean a few times which doesn't seem to have cured it.

Here's the weird thing, every single time the code is triggered and the dash warning lights up, I'm low on fuel, like on the first notch. From what I know, this has to be a coincidence, but it's not. I tested it a couple of times and sure enough as soon as the fuel runs low the check engine light comes on.

Any ideas what could be causing this ??
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Hi kowsmou.
If this is as predictable as you say, plug in a good scan tool, pull up all 4 O2 sensors, and short term and long term fuel trim and record exactly what changes when the low fuel light comes on.
It may be the case that nothing does but something is causing the post CAT O2 sensors to not like what it sees. Live data willl pinpoint this without fail.

Also, you say he's going to change the pre cat sensor. Which one? Which bank? There are 4 sensors on this engine, even though it is the 2.0.
Which bank is the Po420 coming up on? I'm assuming its bank 1, since bank 2 is sometimes a Po430.

Andy.
 
Oh, the reason the money light comes on when the low fuel does? The ECU may have a strategy that looks at various sensors at specific times. It may be programmed to look at the fuel level at the same time as it calculates the CAT efficiency from the data. Which could explain why they come on together.
Can't be certain but it's possible. Only the engineers who designed the system will know.
But for us, it doesn't matter. Concentrate on the Po420 and the reason its coming on first. Don't get bogged down by possible coincidences or not.

Andy.
 
kowsmou said:
srhutch said:
Real trait of these 2.0i zed s :cry:
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How do you mean sorry? I've not had one long!
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I will second this - I owned a 2.0 Zed for 6 months and in the last 2 months of ownership I had pretty much the same issue as you. After 2 or 3 hundred miles the engine warning light would come on - I'd clear it with a very basic OBD (can't remember the exact code but I know they weren't helpful in narrowing down the exact fault), and then after another roughly 2 or 3 hundred miles it would come on again. It didn't effect the driving experience and I never got to the bottom of it as I traded it in.
 
Strange they are going for the pre-cat sensors when the fault is catalytic efficiency. It's the post-cat o2 sensors that determine the cat efficiency. The pre-cat sensors are used in closed loop mixture adjustment at part throttle opening (steady state cruising).
 
ph001 said:
Strange they are going for the pre-cat sensors when the fault is catalytic efficiency. It's the post-cat o2 sensors that determine the cat efficiency. The pre-cat sensors are used in closed loop mixture adjustment at part throttle opening (steady state cruising).
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This..

Unless the ecu is comparing pre and post readings in order to determine efficiency?

Mike
 
Even the basic code readers like Carly can display lambda voltage - the pre-cat one should be oscillating between approx 0.8V and 0.2V every second or so. The post cat one should be relatively stable. If the post-cat one is oscillating much like the pre-cat one then it means that your cat is not doing it's job. If the voltage on the post-cat one is stable but at the wrong voltage (should be about 0.1 or 0.2V) then it's more likely a duff sensor.
 
Could be way off here but fuel pump?
Maybe slightly dropping fuel pressure causing a "Cough" then throwing a lambda code because the values have drastically change from pre>post cat?
 
coupedan said:
Could be way off here but fuel pump?
Maybe slightly dropping fuel pressure causing a "Cough" then throwing a lambda code because the values have drastically change from pre>post cat?
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I think that would throw up a lean mixture code :?
Rob
 
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