What it means
The switched-battery input to the ECM/PCM is present, but the level or transient behaviour is out of plausibility — sagging during cranking, slow ramp-up after main-relay close, or noisy. Often caused by aged main relay contacts or a weak battery feed under load.
At a glance
- Usually lights the check-engine light
How it works
The ECM or PCM receives switched battery power through a feed circuit that must remain stable as the vehicle starts and runs. P2506 sets when the module sees that input behaving outside its expected range or pattern, such as sagging or arriving inconsistently, rather than simply being absent.
Common causes
Most likely first.
- Poor connection or excessive resistance in the ECM/PCM power feed
- Weak battery
- Faulty relay or wiring
Symptoms
- Check-engine light
- Intermittent stalling or no-start
- Engine may cut out during cranking or operation
How to diagnose it
- Save freeze-frame data and note whether the fault occurred during cranking, relay activation, or engine operation.
- Inspect the battery terminals, main relay, module power-feed wiring, and ECM/PCM connector pins for corrosion, looseness, or heat damage.
- Check battery condition and observe the module's power input during cranking and relay activation, comparing behavior with the manufacturer's specification.
- Test voltage drop across the positive feed, relay contacts, and grounds under load to find excessive resistance.
- If the supply path tests correctly, check relay operation and wiring for noise or intermittent behavior while monitoring the input signal.
Common fixes
- Clean and tighten battery terminals and power-feed connections.
- Repair high-resistance or damaged wiring and ground connections in the ECM/PCM supply circuit.
- Replace a weak battery if testing confirms it cannot maintain the required supply under load.
- Replace a faulty main relay if its contacts cause an abnormal voltage drop or delayed supply.
Can I keep driving?
The engine may stall or fail to start if the module supply drops, so avoid driving when the fault causes cutouts or difficult cranking. A repeat voltage sag can also lead to further loss of electrical functions.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.