A collapsed 5 V reference can make a vehicle look as if several unrelated sensors failed simultaneously. MAP, fuel pressure, A/C pressure and other transducers may share the same regulator inside the ECU. If one sensor or wire shorts the reference to ground, every sensor on that branch can report implausibly low or missing values.
The key clue is commonality. If multiple 5 V sensor codes appear together and the reference measures near zero, unplug sensors on the shared branch one at a time while monitoring the reference. If 5 V returns when one component or branch is disconnected, you have isolated the area. Do this only with correct service information so you do not disconnect safety-critical components casually.
A short to battery voltage is a different and potentially more serious fault. Do not feed external voltage into a reference line to ‘see what happens.’ The regulator and ECU inputs are not designed for that test.
After isolating the problem, inspect the harness where it can rub against brackets, engine covers or metal pipes. Intermittent reference shorts are often movement- or heat-related.
Figure: Shared 5-Volt Reference Faults