diff --git a/AGENTS.md b/AGENTS.md
index 67dba00..dcb6ec0 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -6,6 +6,7 @@ This repository contains a small ESP8266/ESP32 Arduino project for monitoring wa
 
 - `arduino/arduino.ino` contains the Arduino sketch and main application logic.
 - `server/log.php` accepts measurement POST requests and appends them as JSON Lines.
+- `server/data-checker.php` is intended for hourly CRON runs and checks whether recent measurements are still being written.
 - `server/functions.inc.php` contains shared PHP helpers, including JSON responses, HTTP requests, and Telegram notifications.
 - `data/YYYY.jsonl` is created at runtime for logged measurements and should not be treated as source code.
 - `README.md` gives the high-level project purpose.
@@ -30,6 +31,7 @@ arduino-cli upload -p COM3 --fqbn esp32:esp32:esp32 arduino
 arduino-cli monitor -p COM3 -c baudrate=115200
 
 php -l server/log.php
+php -l server/data-checker.php
 php -l server/functions.inc.php
 ```
 
@@ -74,10 +76,13 @@ Before committing firmware changes:
 Before committing server changes:
 
 - Run `php -l server/log.php`.
+- Run `php -l server/data-checker.php` when the CRON checker changes.
 - Run `php -l server/functions.inc.php` when shared helpers change.
 - Test that non-POST requests return HTTP 405 with `Allow: POST`.
 - Test a form POST such as `level1=1&level2=0&level3=0&level4=1`.
 - Verify that a single compact JSON object is appended to `data/YYYY.jsonl`.
+- Verify the CRON checker stays silent when `data/YYYY.jsonl` is missing, empty, younger than 45 minutes, or older than 3 hours and 45 minutes.
+- Verify the CRON checker walks backward to the nearest valid JSON line when the last line is invalid.
 - For notification changes, verify that Telegram messages are sent only when `level1..level4` differ from the previous saved measurement.
 - Verify invalid level combinations such as `0100`, `1010`, and `1101` produce the measurement error notification without causing an HTTP error.
 
diff --git a/server/data-checker.php b/server/data-checker.php
new file mode 100644
index 0000000..4f98eb8
--- /dev/null
+++ b/server/data-checker.php
@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__.'/functions.inc.php';
+
+const MIN_AGE_SECONDS = 45 * 60;
+const MAX_AGE_SECONDS = (3 * 60 * 60) + (45 * 60);
+
+function loadLastValidRecord(string $filename): ?array
+{
+	if (!is_file($filename)) {
+		return null;
+	}
+
+	$lines = file($filename, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
+	if ($lines === false) {
+		return null;
+	}
+
+	// Walk backwards so a partially written or invalid last line does not break the check.
+	for ($i = count($lines) - 1; $i >= 0; $i--) {
+		$record = json_decode($lines[$i], true);
+		if (is_array($record) && json_last_error() === JSON_ERROR_NONE) {
+			return $record;
+		}
+	}
+
+	return null;
+}
+
+function parseRecordTime(array $record): ?DateTimeImmutable
+{
+	if (!isset($record['timestamp']) || !is_string($record['timestamp'])) {
+		return null;
+	}
+
+	try {
+		return new DateTimeImmutable($record['timestamp']);
+	} catch (Throwable $e) {
+		error_log('Invalid data timestamp: ' . $e->getMessage());
+		return null;
+	}
+}
+
+function formatLevels(array $record): string
+{
+	$values = [];
+	foreach (['level1', 'level2', 'level3', 'level4'] as $name) {
+		$values[] = $name . '=' . (isset($record[$name]) ? (int) $record[$name] : 0);
+	}
+
+	return implode(', ', $values);
+}
+
+$record = loadLastValidRecord(DATA_JSONL_FILEPATH);
+if ($record === null) {
+	exit(0);
+}
+
+$recordTime = parseRecordTime($record);
+if ($recordTime === null) {
+	exit(0);
+}
+
+$ageSeconds = time() - $recordTime->getTimestamp();
+if ($ageSeconds <= MIN_AGE_SECONDS || $ageSeconds >= MAX_AGE_SECONDS) {
+	exit(0);
+}
+
+$message = sprintf(
+	'Nebola zaznamenaná nová hodnota hladiny kondenzu, posledná získaná hodnota je z %s (%s). Toto upozornenie príde len 3x, potom sa už nekontroluje.',
+	$recordTime->format('Y-m-d H:i:s'),
+	formatLevels($record)
+);
+
+notifyUsers($message);
diff --git a/server/functions.inc.php b/server/functions.inc.php
index 9ffa30a..f45b392 100644
--- a/server/functions.inc.php
+++ b/server/functions.inc.php
@@ -1,5 +1,7 @@
 <?php
 
+define('DATA_JSONL_FILEPATH', __DIR__ . DIRECTORY_SEPARATOR . '..' . DIRECTORY_SEPARATOR . 'data' . DIRECTORY_SEPARATOR . date('Y') . '.jsonl');
+
 function respondJson(int $statusCode, array $payload): void
 {
 	http_response_code($statusCode);
diff --git a/server/log.php b/server/log.php
index cca29b5..f1a53fe 100644
--- a/server/log.php
+++ b/server/log.php
@@ -125,7 +125,7 @@ if (!is_dir($dataDir)) {
 	}
 }
 
-$filename = $dataDir . DIRECTORY_SEPARATOR . date('Y') . '.jsonl';
+$filename = DATA_JSONL_FILEPATH;
 $json = json_encode($record);
 if ($json === false) {
 	respondJson(500, [