During an inspection, what indicates that the vehicle is not ready for testing?

A low coolant temperature during an inspection often indicates that the engine has not yet reached its optimal operating temperature. This can be significant because many emissions control systems, including the catalytic converter and oxygen sensors, require the engine to be within a specific temperature range to function properly. If these systems are not operating correctly, the vehicle may produce higher emissions than allowed, thus indicating that it is not yet ready for a smog check.

In contrast, having all indicators on the dash green, high battery voltage, and operational lights suggest that the vehicle's systems are functioning adequately and may not directly relate to the readiness for an emissions test. Operational indicators usually reflect that there are no critical warning issues, while high battery voltage can be misleading if the engine is not warmed up. Hence, the correct choice focuses on the importance of engine temperature as a readiness factor for emissions testing.