diff --git a/src/specificClass.js b/src/specificClass.js
index d34170d..073d3f3 100644
--- a/src/specificClass.js
+++ b/src/specificClass.js
@@ -286,7 +286,7 @@ class PumpingStation extends BaseDomain {
     const measurementType = child.config.asset.type;
     const eventName = `${measurementType}.measured.${position}`;
 
-    child.measurements.emitter.on(eventName, (eventData = {}) => {
+    const handle = (eventData = {}) => {
       this.logger.debug(
         `Measurement update ${eventName} <- ${eventData.childName || child.config.general.name}: ${eventData.value} ${eventData.unit}`
       );
@@ -297,7 +297,21 @@ class PumpingStation extends BaseDomain {
       this.measurements.type(measurementType).variant('measured').position(position)
         .value(eventData.value, eventData.timestamp, eventData.unit);
       this.measurementRouter.route(measurementType, eventData.value, position, eventData);
-    });
+    };
+
+    child.measurements.emitter.on(eventName, handle);
+
+    // Seed from the child's current value. The emitter only delivers FUTURE
+    // updates, so a parent that registers after the child already emitted
+    // (e.g. a once-only inject that fired during startup before this
+    // subscription existed) would otherwise never see that value. Replaying
+    // the last sample makes a late subscriber pick up the present state.
+    const series = child.measurements
+      .type(measurementType).variant('measured').position(position).get?.();
+    const sample = series?.getLaggedSample?.(0);
+    if (sample && sample.value != null) {
+      handle({ ...sample, childName: child.config.general.name });
+    }
   }
 
   _subscribePredictedFlow(child) {
diff --git a/test/basic/replay-on-subscribe.basic.test.js b/test/basic/replay-on-subscribe.basic.test.js
new file mode 100644
index 0000000..7268d55
--- /dev/null
+++ b/test/basic/replay-on-subscribe.basic.test.js
@@ -0,0 +1,81 @@
+// Late-subscriber replay: a measurement child that already holds a value when
+// the pumpingStation registers it (e.g. a once-only inject that fired during
+// startup before the parent subscribed) must still surface on Port 0. The
+// emitter only delivers future updates, so _subscribeMeasurement seeds from the
+// child's current sample.
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const EventEmitter = require('node:events');
+
+const PumpingStation = require('../../src/specificClass');
+const { MeasurementContainer, configManager } = require('generalFunctions');
+
+function makePsConfig() {
+  const cm = new configManager();
+  return cm.buildConfig('pumpingStation', { name: 'PS' }, 'ps-replay', {
+    basin: { volume: 50, height: 5, inflowLevel: 3, outflowLevel: 0.2, overflowLevel: 4.5 },
+    hydraulics: { minHeightBasedOn: 'outlet' },
+    control: {
+      mode: 'levelbased',
+      allowedModes: new Set(['levelbased']),
+      levelbased: { minLevel: 1, startLevel: 2, maxLevel: 4, curveType: 'linear' },
+    },
+    safety: {},
+  });
+}
+
+function makeFlowMeasurementChild(id = 'meas-replay') {
+  const measurements = new MeasurementContainer({ autoConvert: true, preferredUnits: { flow: 'm3/s' } });
+  assert.ok(typeof measurements.emitter?.on === 'function');
+  return {
+    id,
+    source: {
+      config: {
+        general: { id, name: id },
+        functionality: { softwareType: 'measurement', positionVsParent: 'upstream' },
+        asset: { type: 'flow' },
+      },
+      measurements,
+    },
+  };
+}
+
+test('value written BEFORE registration is replayed on subscribe (once-inject timing)', () => {
+  const ps = new PumpingStation(makePsConfig());
+  const child = makeFlowMeasurementChild();
+
+  // Child already holds a value — emitted into the void before the parent existed.
+  child.source.measurements
+    .type('flow').variant('measured').position('upstream')
+    .value(50, Date.now(), 'm3/h');
+
+  // Parent registers AFTER the value is present. Without replay it would only
+  // catch future emits and surface nothing.
+  ps.childRegistrationUtils.registerChild(child.source, 'upstream');
+
+  const out = ps.getOutput();
+  const upstreamKeys = Object.keys(out).filter((k) => k.startsWith('flow.measured.upstream'));
+  assert.ok(upstreamKeys.length > 0, 'parent must surface flow.measured.upstream.* after late subscribe');
+});
+
+test('no stored value → nothing replayed, no crash', () => {
+  const ps = new PumpingStation(makePsConfig());
+  const child = makeFlowMeasurementChild('empty-child');
+  // Register with an empty child container; replay must be a safe no-op.
+  assert.doesNotThrow(() => ps.childRegistrationUtils.registerChild(child.source, 'upstream'));
+  const out = ps.getOutput();
+  const upstreamKeys = Object.keys(out).filter((k) => k.startsWith('flow.measured.upstream'));
+  assert.equal(upstreamKeys.length, 0, 'no upstream key when child has no value');
+});
+
+test('future emits still delivered after subscribe (listener intact)', () => {
+  const ps = new PumpingStation(makePsConfig());
+  const child = makeFlowMeasurementChild('streaming-child');
+  ps.childRegistrationUtils.registerChild(child.source, 'upstream');
+  // Emit AFTER registration — the normal streaming-sensor path.
+  child.source.measurements.type('flow').variant('measured').position('upstream').value(30, Date.now(), 'm3/h');
+  const out = ps.getOutput();
+  const upstreamKeys = Object.keys(out).filter((k) => k.startsWith('flow.measured.upstream'));
+  assert.ok(upstreamKeys.length > 0, 'normal post-subscribe emit still surfaces');
+});