A 2017 Land Rover Discovery gets pushed into the bay with a crank but no start condition and a pair of codes that point in two very different directions: P0A0F97 and U008087. When the engine control module and the body control module stop talking, the immobilizer steps in, and your bay is instantly tied up. The technician working on this vehicle noted simply that the car would not start.
Decoding the communication breakdown
The L462 platform is complex to diagnose. When you pull P0A0F97, it acts as a generic flag that the engine failed to start. On its own, it tells you very little about the root cause. But paired with U008087, which flags a Vehicle Communication Bus A Performance issue, the diagnostic picture sharpens considerably.
The engine cranks but will not fire, and the network connecting the Powertrain Control Module (PCM) to the Body Control Module (BCM) is degraded or completely down. Shops often lose bay hours here by chasing the engine fault and the network fault as two separate, unrelated problems. The diagnostic platform we build analyzes these codes together to map out a logical testing path.
The hidden voltage drop
Our engine ranked a power supply or grounding problem as the most likely root cause, giving it a 90 percent probability with a high severity rating. The L462 architecture is extremely sensitive to low voltage or an unclean ground connection. If the main ground cable connecting the engine to the chassis is compromised, or if there is corrosion at the battery terminals or loose connections within the battery distribution box, the starter motor will pull all available current. This creates a massive voltage drop across the entire electrical system.
When the voltage drops below a critical threshold during cranking, the CAN-bus communication fails. The U008087 code triggers because the PCM and BCM can no longer talk to each other. Without that active network link, the PCM never receives the immobilizer authorization from the BCM. Without authorization, the PCM refuses to pulse the fuel injectors, resulting in the P0A0F97 code. This problem often occurs suddenly during driving, which strongly indicates a sudden loss of a good ground connection or a broken cable.
To prove this out, the required tests are entirely electrical. The platform suggested starting with a heavy battery load test. Next, a technician must measure the voltage drop across the main engine ground cable while cranking. Finally, checking the actual voltage arriving at the PCM pins during a start attempt will confirm if the module is starving for power.
Inspecting the physical network layer
If the voltage and grounds are solid, the platform ranked a physical CAN-bus communication fault as the next most likely culprit at 80 percent probability. The twisted pair wiring on Bus A is vulnerable to physical damage. Water ingress at the PCM connectors is a known killer on modern networks, and pin corrosion can add enough resistance to degrade the signal. If the BCM is trying to tell the PCM that the immobilizer is unlocked, but the message degrades into noise, the PCM will keep the injectors shut off.
The diagnostic path here moves away from the multimeter and towards visual and data analysis. The first step is a thorough visual inspection of the CAN-bus wiring harnesses and the module connectors. If everything looks dry and intact, a resistance measurement of the network comes next. Finally, a technician should monitor the live data streams using a scan tool to check the immobilizer status in both the BCM and the PCM. If they disagree, the network is breaking the message.
Ruling out internal module failures
Ranked third at 70 percent was an internal failure of the PCM itself. Sometimes a module simply fails to process data internally. If the PCM stops functioning correctly, it cannot perform the calculations required for fuel injection and timing, even though it sees the crankshaft spinning. A dead processor inside the PCM will also drop off the CAN-bus, immediately throwing the U008087 network code.
Before condemning a very expensive module, the testing must be rigorous. The platform advised checking the main power and ground supplies directly at the PCM one more time. Then, using OEM diagnostic routines, a technician should poll the PCM status. Checking the reference voltages sent out to engine sensors is a great way to see if the internal power supply of the module is still alive.
The diesel fuel system trap
Finally, at 60 percent probability, the platform suggested a problem with the high-pressure side of the fuel system. This is a diesel engine, so if it cranks and refuses to start, lack of rail pressure is always on the table. A sudden failure could be a sheared high-pressure fuel pump, a completely blocked fuel filter, injectors that are stuck open and bleeding off pressure, or a dead fuel pressure sensor lying to the PCM.
However, testing the fuel system is labor intensive. You have to check the low-pressure lift pump, measure common rail pressure, and perform injector leak-back tests. This is exactly why analyzing multiple codes together matters so much in a busy workshop. If a technician ignores the network code and focuses only on the engine start failure, they might spend hours tearing into the diesel fuel system before realizing the injectors are perfectly fine, but the PCM is actively holding them shut due to a network error.
Next steps for your workshop
Chasing a crank and no-start condition on a modern network is about knowing what to test first. Have you seen these L462 ground issues masquerading as network failures in your own workshop?
Reference pages for the codes in this article
- P0A0F, what we measure for this code across real diagnostics
