Hey guys,
Recently the car went into limp with a P1602 fault code and I would like to know what can trigger that on the ECU. I have checked battery voltage and alternator voltage and can find no failures. 12.4 V on battery and charging with some 14 V. The fault is constant and that should hopefully make it more easy to find the culprit.
I have a feeling that it might be caused by a broken wire/connection to the ECU - and would very much like to know which pins on the D4 or M10 that is used by the ECU (or BC ?) to kick of such a fault. I also wonder where the A/D converter thing is located. Thanks.

P1602 A/D converter
P1602 Battery voltage
P1602 Voltage circuit
P1602 Voltage regulator

Well well well. Did a full throttle relearn - 1 minute with ignition off then 1 minute with ignition on - did that 2 times - then started the engine - still engine fault but now I could go beyond 1100 rev. so took it to 6000 for some 15 seconds and turned the engine off. Waitet for one minute. Turned the engine on. Then surprisingly no fault. No P1602. Then I let the engine idle for 2 minutes and took it for a spin with max power and screaming tyres. But no faults. I will replace the alternator next time - I believe the high rev. cured the fault for some time. At least the car is now on the road again.

"P1602 - Communication between immobilizer module and engine control unit"

from https://www.fiatforum.com/tech-talk/266337-fiat-specific-ecu-eobd-obd-dtc-error-codes-list-full.html

Not sure that fits your experience though

Thanks a lot for your advice - and yes that could very easily be the case hence I did have some problems starting the car - took me several attempts. I did have to wait a few seconds before turning the key with ignition on. And the voltage seems fine so immobilizer communication issue could be the reason for the failure. Does anyone know how to fix that thing ?

There are a lot of connections from the body computer to the ECU - I have cleaned the infamous D4 as a start but still I have the P1602.

Well well well the body computer seems to have some connection problems to the ECU the big connectors from the computer do have a weakness I will extract the computer from the dash board and look into each of them.

let me know how the job was, I have U1702 code pretty much constantly so will probably have to do the same jobs soon

Fairly easy job just follow this guide U1702 cant find that in the bug list. Indeed this car does have some troubles with the harness but using a fine contact spray that really deoxidize the connection pins will help a lot.

torinst

Have you relocated the 1.6 ECU as it suffers from engine heat soak and causes the pins to drop which gives various errors ?

Yes I have inspected the ECU and no pins dropped. Also inspected the infamous D4 connector. Yesterday I worked with the bodycomputer BCM. I could provoke the 1602 failure to come and go just by squeezing/tuching the BCM box gently. The BCM actually consists of 2 boxes put together by a big connector plug and that connector does have my great interest. Will come back with some more info when or if I am lucky to find the culprit.

Today the bodycomputer was split apart and no visual defects observed - but it seems more and more likely that the board does have a problem - might be a weak soldering - so I ended up with 'baking' the board in the owen - 140 degrees celcius for 20 mins - tomorrow I will mount it in the car.

After having baked the BCM the car runs fine in 20 mins then again P1602. But the BCM is no more sensitive to any handeling.The question is - does the P1602 fault code really apply to the immobilizer communication between BCM and ECU ? So far the codes in all other aspects, eg an accelerator failure I have had, stems just fine with another list showing that is a problem related to eg. voltage regulator https://www.automotive-manuals.net/fiat-fault-codes/. I am happy that the Stilo is an extra car we have if we need one