“Steve, my Volt won’t run on electric anymore. Must be the high-voltage battery.” It’s a complaint we hear often, but usually that’s not the real problem. “Bring it over, let’s take a look.”

As expected, the check engine light is on, and when we plug in to read the codes, we get a long list. To narrow things down, we record them, clear the system, and take the car for a road test. It usually takes several test drives before a code and matching symptom reappear. Eventually, we get P0964: Transmission Control Solenoid Valve 2 Control Circuit Open.

Good, we have a code. Time to follow the diagnostic chart. Step one: discharge the high-voltage system to avoid any nasty surprises. Step two: remove the front cover of the transmission, where all the solenoids are located. Right away, something doesn’t look right. The fluid isn’t red. It’s black, very dark.

Replace or Rethink

Next steps call for circuit measurements and ultimately lead to replacing the solenoid block. Will that fix the issue? No. Once again, before diving into lengthy checks, a quick look at the technical service bulletins would have shown that this code can also be triggered by a wiring issue inside the transmission. If the harness dates from before December 2, 2018, it’s likely the culprit. That was the case here.

After replacing both the solenoid block and the wiring harness, the customer was back on the road with full electric mode restored.