diff --git a/README.md b/README.md
index 7cd0664..3565a94 100644
--- a/README.md
+++ b/README.md
@@ -175,7 +175,7 @@ Packet layout:
 
 LoRa radio settings:
 - Frequency: **433 MHz** or **868 MHz** (set by build env via `LORA_FREQUENCY_HZ`)
-- SF11, BW 125 kHz, CR 4/5, CRC on, Sync Word 0x34
+- SF12, BW 125 kHz, CR 4/5, CRC on, Sync Word 0x34
 
 ## Data Format
 MeterData JSON (sender + MQTT):
@@ -205,6 +205,10 @@ Fixed header (little-endian):
 - `dt_s` u8 (seconds, >0)
 - `n` u8 (sample count, <=30)
 - `battery_mV` u16
+- `err_m` u8 (meter read failures, sender-side counter)
+- `err_d` u8 (decode failures, sender-side counter)
+- `err_tx` u8 (LoRa TX failures, sender-side counter)
+- `err_last` u8 (last error code: 0=None, 1=MeterRead, 2=Decode, 3=LoraTx)
 
 Body:
 - `E0` u32 (absolute energy in Wh)
@@ -219,6 +223,7 @@ Body:
 Notes:
 - Receiver reconstructs timestamps from `t_last` and `dt_s`.
 - Total power is computed on receiver as `p1 + p2 + p3`.
+- Sender error counters are carried in the batch header and applied to all samples.
 
 ## Device IDs
 - Derived from WiFi STA MAC.
@@ -250,6 +255,7 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
   - `/`: status overview
   - `/wifi`: WiFi/MQTT/NTP config (AP and STA)
   - `/sender/<device_id>`: per-sender details
+- Sender IDs on `/` are clickable (open sender page in a new tab).
 
 ## MQTT
 - Topic: `smartmeter/<deviceId>/state`
@@ -272,6 +278,8 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 - `lilygo-t3-v1-6-1-test`: test build with `ENABLE_TEST_MODE`
 - `lilygo-t3-v1-6-1-868`: production build for 868 MHz modules
 - `lilygo-t3-v1-6-1-868-test`: test build for 868 MHz modules
+- `lilygo-t3-v1-6-1-payload-test`: build with `PAYLOAD_CODEC_TEST`
+- `lilygo-t3-v1-6-1-868-payload-test`: 868 MHz build with `PAYLOAD_CODEC_TEST`
 
 ## Config Knobs
 Key timing settings in `include/config.h`:
@@ -283,6 +291,7 @@ Key timing settings in `include/config.h`:
  - `BATCH_RETRY_POLICY` (keep or drop on retry exhaustion)
  - `SERIAL_DEBUG_MODE` / `SERIAL_DEBUG_DUMP_JSON`
  - `LORA_SEND_BYPASS` (debug only)
+ - `ENABLE_SD_LOGGING` / `PIN_SD_CS`
 
 ## Limits & Known Constraints
 - **Compression**: MeterData uses lightweight RLE (good for JSON but not optimal).
@@ -292,6 +301,14 @@ Key timing settings in `include/config.h`:
 - **OLED**: no hardware reset line is used (matches working reference).
 - **Batch ACKs**: sender waits for ACK after a batch and retries up to `BATCH_MAX_RETRIES` with `BATCH_ACK_TIMEOUT_MS` between attempts.
 
+## SD Logging (Receiver)
+Optional CSV logging to microSD (FAT32) when `ENABLE_SD_LOGGING = true`.
+
+- Path: `/dd3/<device_id>/YYYY-MM-DD.csv`
+- Columns:
+  `ts_utc,p_w,p1_w,p2_w,p3_w,e_kwh,bat_v,bat_pct,rssi,snr,err_m,err_d,err_tx,err_last`
+- `err_last` is written as text (`meter`, `decode`, `loratx`) only on the last sample of a batch that reports an error.
+
 ## Files & Modules
 - `include/config.h`, `src/config.cpp`: pins, radio settings, sender IDs
 - `include/data_model.h`, `src/data_model.cpp`: MeterData + ID init
diff --git a/include/config.h b/include/config.h
index 250e727..a3bab78 100644
--- a/include/config.h
+++ b/include/config.h
@@ -73,9 +73,11 @@ constexpr uint8_t BATCH_QUEUE_DEPTH = 10;
 constexpr BatchRetryPolicy BATCH_RETRY_POLICY = BatchRetryPolicy::Keep;
 constexpr uint32_t WATCHDOG_TIMEOUT_SEC = 120;
 constexpr bool ENABLE_HA_DISCOVERY = true;
-constexpr bool SERIAL_DEBUG_MODE = true;
-constexpr bool SERIAL_DEBUG_DUMP_JSON = true;
+constexpr bool SERIAL_DEBUG_MODE = false;
+constexpr bool SERIAL_DEBUG_DUMP_JSON = false;
 constexpr bool LORA_SEND_BYPASS = false;
+constexpr bool ENABLE_SD_LOGGING = false;
+constexpr uint8_t PIN_SD_CS = 25;
 
 constexpr uint8_t NUM_SENDERS = 1;
 inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = {
diff --git a/src/main.cpp b/src/main.cpp
index dcbaf46..9dc364f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -13,6 +13,7 @@
 #include "web_server.h"
 #include "display_ui.h"
 #include "test_mode.h"
+#include "sd_logger.h"
 #include <stdarg.h>
 #include <math.h>
 #ifdef ARDUINO_ARCH_ESP32
@@ -625,6 +626,7 @@ void setup() {
     update_battery_cache();
   } else {
     power_receiver_init();
+    sd_logger_init();
     wifi_manager_init();
     init_sender_statuses();
     display_set_sender_statuses(g_sender_statuses, NUM_SENDERS);
@@ -806,6 +808,7 @@ static void receiver_loop() {
           data.link_valid = true;
           data.link_rssi_dbm = pkt.rssi_dbm;
           data.link_snr_db = pkt.snr_db;
+          sd_logger_log_sample(data, data.last_error != FaultType::None);
           for (uint8_t i = 0; i < NUM_SENDERS; ++i) {
             if (pkt.device_id_short == EXPECTED_SENDER_IDS[i]) {
               data.short_id = pkt.device_id_short;
@@ -897,6 +900,7 @@ static void receiver_loop() {
             data.err_decode = batch.err_d;
             data.err_lora_tx = batch.err_tx;
             data.last_error = static_cast<FaultType>(batch.err_last);
+            sd_logger_log_sample(data, (s + 1 == count) && data.last_error != FaultType::None);
           }
 
           if (sender_idx >= 0) {
diff --git a/src/sd_logger.cpp b/src/sd_logger.cpp
new file mode 100644
index 0000000..ceabfcd
--- /dev/null
+++ b/src/sd_logger.cpp
@@ -0,0 +1,114 @@
+#include "sd_logger.h"
+#include "config.h"
+#include <SD.h>
+#include <SPI.h>
+#include <time.h>
+
+static bool g_sd_ready = false;
+
+static const char *fault_text(FaultType fault) {
+  switch (fault) {
+    case FaultType::MeterRead:
+      return "meter";
+    case FaultType::Decode:
+      return "decode";
+    case FaultType::LoraTx:
+      return "loratx";
+    default:
+      return "";
+  }
+}
+
+static bool ensure_dir(const String &path) {
+  if (SD.exists(path)) {
+    return true;
+  }
+  return SD.mkdir(path);
+}
+
+static String format_date_utc(uint32_t ts_utc) {
+  time_t t = static_cast<time_t>(ts_utc);
+  struct tm tm_utc;
+  gmtime_r(&t, &tm_utc);
+  char buf[16];
+  snprintf(buf, sizeof(buf), "%04d-%02d-%02d",
+           tm_utc.tm_year + 1900,
+           tm_utc.tm_mon + 1,
+           tm_utc.tm_mday);
+  return String(buf);
+}
+
+void sd_logger_init() {
+  if (!ENABLE_SD_LOGGING) {
+    g_sd_ready = false;
+    return;
+  }
+  g_sd_ready = SD.begin(PIN_SD_CS);
+}
+
+bool sd_logger_is_ready() {
+  return g_sd_ready;
+}
+
+void sd_logger_log_sample(const MeterData &data, bool include_error_text) {
+  if (!g_sd_ready || data.ts_utc == 0) {
+    return;
+  }
+
+  String root_dir = "/dd3";
+  if (!ensure_dir(root_dir)) {
+    return;
+  }
+
+  String sender_dir = root_dir + "/" + String(data.device_id);
+  if (!ensure_dir(sender_dir)) {
+    return;
+  }
+
+  String filename = sender_dir + "/" + format_date_utc(data.ts_utc) + ".csv";
+  bool new_file = !SD.exists(filename);
+  File f = SD.open(filename, FILE_APPEND);
+  if (!f) {
+    return;
+  }
+
+  if (new_file) {
+    f.println("ts_utc,p_w,p1_w,p2_w,p3_w,e_kwh,bat_v,bat_pct,rssi,snr,err_m,err_d,err_tx,err_last");
+  }
+
+  f.print(data.ts_utc);
+  f.print(',');
+  f.print(data.total_power_w, 1);
+  f.print(',');
+  f.print(data.phase_power_w[0], 1);
+  f.print(',');
+  f.print(data.phase_power_w[1], 1);
+  f.print(',');
+  f.print(data.phase_power_w[2], 1);
+  f.print(',');
+  f.print(data.energy_total_kwh, 3);
+  f.print(',');
+  f.print(data.battery_voltage_v, 2);
+  f.print(',');
+  f.print(data.battery_percent);
+  f.print(',');
+  f.print(data.link_rssi_dbm);
+  f.print(',');
+  if (isnan(data.link_snr_db)) {
+    f.print("");
+  } else {
+    f.print(data.link_snr_db, 1);
+  }
+  f.print(',');
+  f.print(data.err_meter_read);
+  f.print(',');
+  f.print(data.err_decode);
+  f.print(',');
+  f.print(data.err_lora_tx);
+  f.print(',');
+  if (include_error_text && data.last_error != FaultType::None) {
+    f.print(fault_text(data.last_error));
+  }
+  f.println();
+  f.close();
+}
diff --git a/src/sd_logger.h b/src/sd_logger.h
new file mode 100644
index 0000000..60b1611
--- /dev/null
+++ b/src/sd_logger.h
@@ -0,0 +1,7 @@
+#pragma once
+
+#include "data_model.h"
+
+void sd_logger_init();
+bool sd_logger_is_ready();
+void sd_logger_log_sample(const MeterData &data, bool include_error_text);
diff --git a/src/web_server.cpp b/src/web_server.cpp
index ac4456e..4682c5d 100644
--- a/src/web_server.cpp
+++ b/src/web_server.cpp
@@ -52,7 +52,8 @@ static String render_sender_block(const SenderStatus &status) {
       }
     }
   }
-  s += "<strong>" + String(status.last_data.device_id) + "</strong>";
+  String device_id = status.last_data.device_id;
+  s += "<strong><a href='/sender/" + device_id + "' target='_blank' rel='noopener noreferrer'>" + device_id + "</a></strong>";
   if (status.has_data && status.last_data.link_valid) {
     s += " RSSI:" + String(status.last_data.link_rssi_dbm) + " SNR:" + String(status.last_data.link_snr_db, 1);
   }