Car cranks but no start

[sol_j]

Did you get this sorted? Fault code points to it being a broken wire on the CANBUS wires going to/from the ECU.

View attachment 49422

One of those should match the PIN out on the ECU. With the battery disconnected, you should have continuity between the pins in the plugs for your ECU (CAN-H & CAN-L), and pins 6 & 14 on the OBD2 socket by the gearstick.

View attachment 49423
Note that if you test them, be sure to use fine tip probes to measure them! Don't push large probes into the sockets as you can spread the pins which will cause you even more headaches. IF you find you have a break in one of those wires, the the break is almost certainly going to be somewhere in the harness between the ECU and wherever it joins the main loom.

Full wiring diagram here https://drive.google.com/file/d/1PysokT7RjHc8Pr_nXBRA-lYKcS8UDmtS/view?usp=drive_link

If canbus looks OK, the check you have power and grounds at the appropriate pins also

Did you get this sorted? Fault code points to it being a broken wire on the CANBUS wires going to/from the ECU.

View attachment 49422

One of those should match the PIN out on the ECU. With the battery disconnected, you should have continuity between the pins in the plugs for your ECU (CAN-H & CAN-L), and pins 6 & 14 on the OBD2 socket by the gearstick.

View attachment 49423
Note that if you test them, be sure to use fine tip probes to measure them! Don't push large probes into the sockets as you can spread the pins which will cause you even more headaches. IF you find you have a break in one of those wires, the the break is almost certainly going to be somewhere in the harness between the ECU and wherever it joins the main loom.

Full wiring diagram here https://drive.google.com/file/d/1PysokT7RjHc8Pr_nXBRA-lYKcS8UDmtS/view?usp=drive_link

If canbus looks OK, the check you have power and grounds at the appropriate pins also

If this is the case what needs to be replaced?

 

[sol_j]

Did you get this sorted? Fault code points to it being a broken wire on the CANBUS wires going to/from the ECU.

View attachment 49422

One of those should match the PIN out on the ECU. With the battery disconnected, you should have continuity between the pins in the plugs for your ECU (CAN-H & CAN-L), and pins 6 & 14 on the OBD2 socket by the gearstick.

View attachment 49423
Note that if you test them, be sure to use fine tip probes to measure them! Don't push large probes into the sockets as you can spread the pins which will cause you even more headaches. IF you find you have a break in one of those wires, the the break is almost certainly going to be somewhere in the harness between the ECU and wherever it joins the main loom.

Full wiring diagram here https://drive.google.com/file/d/1PysokT7RjHc8Pr_nXBRA-lYKcS8UDmtS/view?usp=drive_link

If canbus looks OK, the check you have power and grounds at the appropriate pins also

Also, this is a quick summary of what has happened so far:
2002 SEAT Leon Cupra (AUQ) – Car cranks but won’t start. No fuel pump sound, no EML light, and no OBD comms. Fuses and relays checked/replaced. Tried known good ECU – same result. Dash lights up (except EML). VCDS previously showed some modules not responding, seemed to get worse over time. Suspect ECU not powering up or wiring fault (power, ground, or CAN issue), not immobiliser.

 

[mty12345]

Thanks for the reply! No I haven’t, waiting on an auto electrician to check it out. Does that mean I’d need a new wiring loom etc? Is this a common problem? Why would this randomly happen? Just old?

No, you can just repair the existing wire, it's just a matter of locating it. Sometimes just old and brittle, sometimes it's possible it can rub on something. Could also be just corrosion somewhere. If you look inside the plug for the ECU, is there any sign of green crusty residue? That's what corrosion looks like. If so you can clean it with isopropyl alcohol and a soft toothbrush.

If other modules were also going offline, could point to the issue being at the can gateway module.

 

[sol_j]

No, you can just repair the existing wire, it's just a matter of locating it. Sometimes just old and brittle, sometimes it's possible it can rub on something. Could also be just corrosion somewhere. If you look inside the plug for the ECU, is there any sign of green crusty residue? That's what corrosion looks like. If so you can clean it with isopropyl alcohol and a soft toothbrush.

If other modules were also going offline, could point to the issue being at the can gateway module

Yeah about 2(?) or so months before this happened the airbag light came on for the drivers side and when I went to try and fix it with a resistor I had no joy. I did notice when I scanned it at that point while doing the airbag light many modules were not being recognised when scanning like the engine control module where on vcds it would say no communications etc. I guess slowly dying(?) also when I did try out the new ecu and took out the old one, tbh it looked clean/ nothing major etc.

 

[sol_j]

No, you can just repair the existing wire, it's just a matter of locating it. Sometimes just old and brittle, sometimes it's possible it can rub on something. Could also be just corrosion somewhere. If you look inside the plug for the ECU, is there any sign of green crusty residue? That's what corrosion looks like. If so you can clean it with isopropyl alcohol and a soft toothbrush.

If other modules were also going offline, could point to the issue being at the can gateway module

Do you have a link/ part number to the can gateway module? Just in case it is that

 

[sol_j]

Do you have a link/ part number to the can gateway module? Just in case it is that

This is what it looked like before

 

[mty12345]

This is what it looked like before

Looks like the gateway itself is fine. It's online and communicating, it just can't speak with the ECU. The wire is likely broken/corroded somewhere between the 2 modules. Either that or ECU is offline due to no power/ground, but my first guess would be the CAN lines. No way of really telling without measuring.

 

[sol_j]

Looks like the gateway itself is fine. It's online and communicating, it just can't speak with the ECU. The wire is likely broken/corroded somewhere between the 2 modules.

Nah this was before, about two weeks before it died. Look at the report the aa said about my car. It doesn’t connect at all now afaik. When the recovery man came to try and fix it he had no connection. Tbh I haven’t tried it since and just waiting for the auto electrician to come out

 

[sol_j]

Looks like the gateway itself is fine. It's online and communicating, it just can't speak with the ECU. The wire is likely broken/corroded somewhere between the 2 modules. Either that or ECU is offline due to no power/ground, but my first guess would be the can lines. No way of really telling without measuring.

Iirc it was like that for many modules later on I’ll post the whole scan