diff --git a/README.md b/README.md
index a289bed..513b4e2 100644
--- a/README.md
+++ b/README.md
@@ -63,9 +63,14 @@ itself — the header order below is only for orientation.
 | RX (UART) | **GPIO17** | P1 |
 | OUT | GPIO16 | P1, optional |
 
-Header order, USB-C end at the top:
+Header order. **Orientation matters: hold the board with the ESP32-S3-WROOM-1
+module and its antenna at the TOP, and both USB-C sockets at the BOTTOM.**
+Pin 1 of each column is the end nearest the module, furthest from the USB
+sockets. (Getting this upside down is an easy way to wire the sensor into the
+PSRAM pins.)
 
 ```
+        [ ESP32-S3-WROOM-1 module + antenna ]   <- TOP
  P1 (left)                          P2 (right)
   1  3V3                             1  GND
   2  3V3                             2  GPIO43  U0TXD -> CH343
@@ -89,8 +94,13 @@ Header order, USB-C end at the top:
  20  GPIO14                         20  GPIO19  USB D-
  21  5V                             21  GND
  22  GND                            22  GND
+        [ USB-C: COM (CH343) | USB ]            <- BOTTOM
 ```
 
+Reference pinout: https://mischianti.org/vcc-gnd-studio-yd-esp32-s3-devkitc-1-clone-high-resolution-pinout-and-specs/
+Note that board also has a plain built-in LED on **GPIO38**, separate from the
+RGB WS2812 on GPIO48 that this firmware drives.
+
 Do not use for I/O: **GPIO35/36/37** (octal PSRAM — the R8 in N16R8),
 **GPIO43/44** (UART0 to the CH343), **GPIO19/20** (native USB), GPIO0/45/46
 (strapping), GPIO48 (RGB LED), GPIO26–32 (flash, not broken out).
diff --git a/src/main.cpp b/src/main.cpp
index 74184c6..03a4623 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -104,6 +104,24 @@ static bool i2cProbe(uint8_t addr) {
 // setup() before WiFi ever starts. Check the lines are actually high with
 // pull-ups on before handing them to Wire.
 static bool i2cBusIdle() {
+  // First, with the internal pull-ups OFF. A powered C4001 breakout has its
+  // own pull-ups, so the lines stay high. If they sag to 0 with nothing
+  // driving them, there is no powered module on these pins at all -- which
+  // separates "sensor wired but mute" from "sensor not actually connected".
+  pinMode(PIN_I2C_SDA, INPUT);
+  pinMode(PIN_I2C_SCL, INPUT);
+  delayMicroseconds(500);
+  int sdaHi = 0, sclHi = 0;
+  for (int i = 0; i < 8; i++) {
+    sdaHi += digitalRead(PIN_I2C_SDA);
+    sclHi += digitalRead(PIN_I2C_SCL);
+    delayMicroseconds(100);
+  }
+  sensor.detectLog += "I2C: external pull-ups sda=" + String(sdaHi) + "/8 scl=" +
+                      String(sclHi) + "/8 " +
+                      ((sdaHi == 8 && sclHi == 8) ? "(module present)"
+                                                  : "(NO powered module on these pins)") + "\n";
+
   pinMode(PIN_I2C_SDA, INPUT_PULLUP);
   pinMode(PIN_I2C_SCL, INPUT_PULLUP);
   delayMicroseconds(200);
@@ -135,7 +153,8 @@ static bool uartSensorPresent() {
   while (Serial1.available()) Serial1.read();  // flush boot noise
 
   // A running C4001 streams frames continuously; nudge it in case it's stopped.
-  Serial1.print("sensorStart\r\n");
+  // No terminator -- DFRobot_C4001_UART::writeReg sends the bare string.
+  Serial1.print("sensorStart");
 
   uint32_t t0 = millis();
   while (millis() - t0 < 1500) {
@@ -171,6 +190,18 @@ static void detectSensor() {
       sensor.detectLog += "I2C: no ACK at 0x" + String(a, HEX) + "\n";
     }
   }
+
+  // Neither expected address answered. Sweep the whole bus so the log can
+  // distinguish "sensor at an unexpected address" from "nothing on the bus
+  // at all" -- which usually means power, swapped SDA/SCL, or the module's
+  // mode switch is set to UART.
+  String found;
+  for (uint8_t a = 1; a < 127; a++) {
+    Wire.beginTransmission(a);
+    if (Wire.endTransmission() == 0) found += " 0x" + String(a, HEX);
+  }
+  sensor.detectLog += found.length() ? "I2C: bus scan ->" + found + "\n"
+                                     : "I2C: bus scan found no devices\n";
   }  // end if (i2cBusIdle())
 
   Serial.println("  probing UART ...");