Engine light strange circumstances

[tony325ci_51]

Speaking of weird engine light (MIL) coming on circumstances, mine 1st came on a few weeks ago.

I bought an ELM-327 usb ODBII interface with software for my laptop off ebay to diagnose the fault code. Hooked it up to the car, read the fault code which claimed the cat wasn't running efficiently. ok, all seemed logical so far.

Filled up with a different brand of petrol, used the laptop to clear the fault code, fine. Checked lambda sensors were operating normally, fine. All seemed well, until;

A couple of weeks later, driving on the exact same stretch of road, same thing, light came on, fault code returned. Repeated above procedure to clear fault code.

Happened twice more since, when travelling at about 60mph under different weather conditions, wet or dry, but (not on any other motorway or road, and I drive all across the UK) ONLY on that EXACT same stretch of road!

Now I'm thinking this road has very high embankments at the sides so perhaps there's some kind of weird air turbulence, wind tunnel effect or vacuum thing going on that is depriving my air intake or blowing into the exhaust, making the car run artificially rich on that stretch of road ?

I don't think it could be a hard fault with the cat or the fault light would come straight back on when the sensors warmed up enough to go closed loop every time the car warmed up, and it doesn't.

I'm puzzled about this one.

[BMWJNKY]

How many miles are on the car? When was the last time the front O2 sensors were changed?

[tony325ci_51]

Originally Posted by BMWJNKY

How many miles are on the car? When was the last time the front O2 sensors were changed?

170k miles.

I don't know when sensors were last changed, but they seem to be giving normal readings when I look at them in the software, voltages fluctuating as expected to constantly alter the fuel trim.

I'd expect to see a different fault code specific to the sensors rather than the cat if the sensors were at fault wouldn't I ?

I'd also guess that the cat efficiency is monitored by the rear post-cat sensors.

The code is P0420 "Catalyst System Efficiency Below Threshold"

This gives some info, (but the diagnosis on voltage is misleading and wrong.)
OBD-II Trouble Code: P0420 Catalyst System Efficiency Below Threshold (Bank 1)

These guys who manufacture sensors seem to explain better how the lambda sensors work, and what they should be doing.
Lambdapower - Lambda Sensor information and supplies - Technical Notes Index Page

The thing is though I'd expect any failure to be consistent, and not just show up on this one particular stretch of road.

[BMWJNKY]

It could be that the load is just perfect to make the light come on. I had a simaliar problem with my Z28 on one streach of highway if I had to do the speed limit and the crusise control was on. Put in a new O2 sensor and it never happened again. BMW says the O2 sensor should be replaced every 80-100K miles and with your car at 170K that is aproaching 2 lifetimes for the sensors. I would replace the sensors and reset the vehicle adaption valuse in the DME and go from there. It may turn out that the cat is bad but 4 O2 sensors are definatly way cheeper than a factory cat and should be replaced at that milage esp. if you don't know when they were last replaced.

[tony325ci_51]

Car has passed another MOT since so cat and emissions must be ok.

The light has only come on about twice in a year or so. As long as I avoid cruise control set to 50 on that stretch of road all seems well. I think what you said about the load is relevant, as cruise control set to 50 on other roads (with different / varying gradients) doesn't light up the MIL.