JTS Lambda Plausibility etc...
I've tried to read up on as many threads as possible before posting this, but haven't found any definite solutions so here goes (sorry about the long 1st post)....
The engine management light keeps coming on on my brother's 2004 156 JTS Selespeed. It does occassionally clear itself, but more often than not stays on until cleared. There are no related driveability issues or symptom I think, apart from a slightly lumpy idle.
The re-occuring fault codes are mainly related to the pre-cat bank one lambda sensor:
P1131 - Bank 1 plausibility
P2251 - Bank 1 pre-cat
P1135 - Bank 1 heater resistor
There has been the occasional bank 2 code as well, but only if we've left the codes uncleared over a longer period of time (i.e. P1161 - Bank 2 pre-cat).
My brother has spent, what I would consider, a small fortune on trying to sort this issue out, but it keeps coming back.
Initially, the pre-cat sensor (bank 1) was replaced by Motor-Mech (Italian car specialist - I use this garage for all my Alfa stuff), but the fix didn't last long. The fault came back not after long, and the garage were unable o ascertain why the fault was recurring as both sensors were working together according to the live data.
It went to another garage (Simply Alfas - brilliant guys - thnk John is a member here?), who carried out mods to the pre-cat sensor plugs (Alfa official mods), and his cured it for a few days. This normally fixes 99.9% of the lambda issues.
As "luck" would have it, we fitted into the 0.1% category, so when the light came back on again, the plugs at the rear of the engine were also hardwired / soldered together. This fix only lasted a day or so too.
Car went back to the garage and it was discovered that the lambda that was replaced was dropping 400mv every now and then, which was when the code was being triggered, i.e. it was a faulty lambda (bad batch?). This was replaced yet again, and everything was fine for a couple of months....
During the winter months, the light came back on once again. The battery was also showing signs of aging (sluggish starts etc) so this was replaced, light reset, and everything was fine for a week, and then the damned light came back on again.
It went back to Simply Alfa's who hooked it up to their computer's and tey reckoned that pre-cat sensor 2 was slightly sluggish, possibly triggering the engine light but reporting the fault as a bank 1 fault. It was thought there wasn't an immedate need to replace the sensor on bank 2.
I conducted a short test few days later whereby we went out for a drive a couple of months ago with AlfaDiag connected just after clearing the codes. The car was in cold start mode.
We were watching both pre-cat sensors on the sensor log / graphs as well as their respective voltages.
Both sensors were going from open loop to closed loop and the graphs were almost identical, as were the voltage graphs. However, after the car warmed up, pre-cat sensor 1 just died (open loop) - the graph was flat, even though the voltage graph was still showing it to be working ok. Although the engine light wasn't triggered, it reported one of the regular codes (P2251?). I cleared it and the sensor came back to life again!
Drove around for quite some time after this, but the sensors worked as normal.
I concluded (rightfully or not) the fault is still pre-cat 1 and NOT pre-cat 2. I also reckon the fault occurs during warm-up and if the code is not cleared, it triggers the other faults (P1131 - Bank 1 plausibility, P1135 - Bank 1 heater resistor, and even P1161 - Bank 2 pre-cat on a couple of occasions) which then cause the light to come on.
What nobody seems to know though, is what the hell is wrong or the fix to this!
The car even went to a Bosch specialist the other week and they want one of the wires was chaffing on the lambda. he checked the lambda (visually) and know it's new, so are also baffled. The was was driving really rough when I got it back though, but I didn't think much of it tining they mut have reset he ECU. Whe the light came back on, he car was now also misfiring on multiple cylinders (P0300) and on cylinder 3 (P0303). I've changed the plugs and the coil pack (cylinder 3), but his was one of the newer coil packs that was changed not to long ago.
The next thing we think is to swap pre-cat sensors around and see if the fault remains at bank 1 (in which case it must be the wiring somewhere) or moves (in which case it will be the sensor itself).
With the modified wiring though, I don't know how to disconnect these so it will have to go back to Simply Alfa's for the swap.
Really confused about this, and would appreciate any help whatso ever (have also had a idle control fault twice as well!)
Last edited by TazzyK; 06-06-10 at 17:14.