P2147 Fuel Injector Group A Supply Voltage Circuit Low: Causes and Diagnosis
P2147 is the generic OBD-II code for “Fuel Injector Group A Supply Voltage Circuit Low”. Modern common-rail ECUs drive their injectors in groups (often called banks), each group sharing a supply stage inside the ECU. P2147 says group A's supply has been dragged below its expected voltage — which is a different, and diagnostically more interesting, fault than a single-injector circuit code. A short to ground anywhere on the group wiring, or one electrically shorted injector, can pull down the supply for every injector in the group — so several cylinders can be affected by a fault that lives in one place.
Technical Background
Which injectors belong to ‘group A’ is defined by the manufacturer and differs between engines — it does not automatically mean cylinders 1 and 2. Identifying group membership from the wiring diagram is the essential first step, because it defines the search area. The exact procedure and specification varies by manufacturer and engine.
The power of a group code is isolation: because the fault sits on a shared supply, disconnecting the group's injectors one at a time while monitoring the circuit typically identifies whether one injector's internal short is loading the supply, or whether the short lives in the loom itself.
Vehicles Commonly Affected
- Any common-rail diesel using grouped injector supply stages — the architecture is near-universal in modern ECUs.
- Often stored alongside its siblings: P2146 (group A circuit/open), P2148 (group A voltage high) or P2149/P2152 (group B equivalents).
Symptoms
- Misfires on more than one cylinder simultaneously — the group signature.
- Possible no-start if the ECU shuts the group down entirely.
- Engine management light, often with limp mode.
- Companion single-cylinder circuit codes for the injectors within the group.
Causes
- Short to ground in the group A supply wiring — chafed loom on a bracket or cover is the classic finding.
- One injector with an internal coil short pulling the shared supply down for the whole group.
- Water ingress or corrosion in a connector creating a leakage path to ground.
- ECU output stage failure on the group supply.
Diagnosis
- 1Get the wiring diagram and identify which cylinders are in group A on this engine — everything else depends on it.
- 2Read all codes: a P2147 plus one specific cylinder circuit code often names the culprit injector immediately.
- 3Inspect the group's loom run for chafing, heat damage and connector corrosion — concentrating on brackets, edges and previous repair sites.
- 4Disconnect the group's injectors one at a time per the manufacturer's isolation procedure, watching whether the supply recovers — recovery on disconnection indicts that injector. The exact procedure and specification varies by manufacturer and engine.
- 5If the fault persists with all group injectors disconnected, the short is in the harness or the ECU — test the loom section before condemning the ECU.
- 6Bench-verify a suspect injector: an internal electrical short is proven conclusively off the vehicle.
Common Mistakes
- Assuming group A means cylinders 1 and 2 without checking the wiring diagram.
- Replacing one injector that carried a companion code without confirming it actually loads the supply — the loom may be the real culprit.
- Missing intermittent chafe shorts that only contact under engine rock — wiggle-test under load conditions.
- Condemning the ECU before the harness has been isolated and proven.
When It's Not the Injectors
- The fault persists with every group injector disconnected — it is the harness or ECU.
- Visible loom damage on the group supply run.
- Wider electrical faults suggesting a general supply or earth problem rather than the injector group.
When Replacement Is Required
Replace an injector only when isolation proves it is loading the group supply — typically an internal coil short confirmed on the bench. The remaining group injectors do not need replacing with it; they were victims of the shared supply, not faulty parts.
Repair
Compatible Engines
Compatible Injectors
Frequently Asked Questions
Which cylinders are in group A?
It is defined by the manufacturer and differs between engines — the wiring diagram for your specific engine is the only reliable source. Getting this wrong wastes the entire diagnosis.
Why do several cylinders misfire from one fault?
Because the injectors share a supply stage. One short — in the loom or inside one injector — pulls the voltage down for every injector in the group. That shared-supply signature is exactly what distinguishes P2147 from single-cylinder codes.
How do I find which injector is shorted?
Isolation: disconnect the group's injectors one at a time per the manufacturer's procedure and watch whether the supply recovers. Recovery on disconnection points at that injector; no recovery points at the harness or ECU.