Check the grounds before you replace a control module
A bad ground imitates almost any module fault, costs nothing to test, and is the cheapest place in the whole job to be wrong.
Every measurement a control module makes is relative to its ground. If the ground is not where the module thinks it is, every reading it takes is offset, every output it commands is wrong, and every one of those errors looks exactly like a failed module.
What a bad ground actually does
A ground connection is supposed to be at zero volts. Corrosion, a loose bolt or a damaged strap turns it into a small resistance, and current flowing through resistance produces a voltage. Now the module's zero is half a volt above the battery's zero. It reads sensors half a volt high, it drives outputs half a volt low, and it reports faults on circuits that are electrically fine.
The symptoms wander, because the offset changes with how much current is flowing. That is why these faults are so often described as intermittent, and why they get chased for hours.
Test with voltage drop, not resistance
This is the part that matters, and it is where most ground testing goes wrong. A resistance reading is taken with no current flowing, and a corroded joint can measure a fraction of an ohm on a meter and still collapse under a real load. The connection has enough contact area for a milliamp and nowhere near enough for ten amps.
- Leave the circuit connected and turn the load on.
- Put one probe on the module's ground terminal and the other directly on the battery negative post.
- Read the voltage between them while the current flows.
- Anything above about 0.1 V on a low-current control ground is a problem. On a high-current path, compare against the manufacturer's figure rather than a rule of thumb.
Where to look on an EV
- Ground straps between the drivetrain and the body — they carry more than people expect and they corrode where they are bolted.
- Grounds behind trim, under carpet and in the boot floor, which is where water ends up.
- Any joint disturbed by earlier repair or by collision work. A ground that was removed once and refitted onto paint is a classic.
- Shared ground points. Several modules on one stud means one bad stud presents as several unrelated faults.
Why it is worth doing first
It takes a few minutes, needs nothing but a multimeter, and rules out the cause that most often imitates the expensive one. Being wrong about a ground costs you time. Being wrong about a control module costs the customer a part they did not need — and the fault is still there when it goes back on the road.