Thanks to Brad Halse from B&L Mechanical Services for sending in this case study
Today's job, a 2010 RC Colorado towed in with the complaint "no-start after washing it". Some time had been spent on it before handballing it to me.
Connect the scan tool and P161B 'Immobiliser Wrong Signal Received' code present. The engine would randomly crank & run for about 2 seconds.
DTC P161B (Flash Code 179)
The ECM communicates with the immobiliser control unit (ICU) to execute the immobiliser function. The ECM transmits a specific request signal to the ICU and the ICU sends back a response signal to the ECM. Both signals are carried out via a controller area network (CAN) communcation bus. If the ECM receives a wrong response signal from the ICU, this DTC will set.
After checking for blown fuses & CAN BUS circuit integrity( both good), it's off to the office for code research.
The factory info gives me the code set criteria, a flow chart & a wiring diagram.Interestingly I looked through the flow chart to see it finishing with the standard ending "Replace ECM".
Now it's worth noting at this point that when engineers design a circuit for a motor vehicle, it hasn't been out in the 'real world' and endured 'real world' testing.Corrosion, cyclic heating, dirt ingress and the list of failures goes on & on.....
The point being? Don't put your faith in these charts.
Out with the circuit diagram I find a wire feeds power from the ignition switch to the ICU (Immobiliser Control Unit).
Dropping the lower shroud from the steering column I find power at the ignition switch (beauty) but not at pin 9 of the ICU.
What's in the middle of these two points? Yep, METER-10amp fuse.
With the cover from the fuse box cleaned up I realised the fuse was actually missing, and what is the next fuse along?
The trusty old Trailer fuse.
.
Clearly the someone has hooked up a trailer with an issue, blown the fuse & grabbed the one next to it.
So once again it came down to having a wiring diagram.... and luckily I didn't have to replace the ECM!
