diff --git a/__tests__/services/api-sensor-attachment-errors.test.ts b/__tests__/services/api-sensor-attachment-errors.test.ts
new file mode 100644
index 0000000..79d7e5c
--- /dev/null
+++ b/__tests__/services/api-sensor-attachment-errors.test.ts
@@ -0,0 +1,386 @@
+import { api } from '@/services/api';
+import * as SecureStore from 'expo-secure-store';
+
+// Mock dependencies
+jest.mock('expo-secure-store');
+jest.mock('expo-file-system');
+
+// Mock fetch globally
+global.fetch = jest.fn();
+
+describe('API - Sensor Attachment Error Handling', () => {
+  const mockBeneficiaryId = '123';
+  const mockWellId = 497;
+  const mockDeviceMac = '142B2F81A14C';
+  const mockDeploymentId = 22;
+  // Use 'robster' to match DEMO_LEGACY_USER in api.ts
+  const mockUserName = 'robster';
+
+  // Create a mock JWT with expiration far in the future
+  const createMockJWT = () => {
+    const header = btoa(JSON.stringify({ alg: 'HS256', typ: 'JWT' }));
+    const payload = btoa(JSON.stringify({
+      sub: mockUserName,
+      exp: Math.floor(Date.now() / 1000) + 3600, // expires in 1 hour
+    }));
+    const signature = 'mock-signature';
+    return `${header}.${payload}.${signature}`;
+  };
+
+  const mockToken = createMockJWT();
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+
+    // Reset SecureStore mock to default implementation
+    // legacyAccessToken needs to be a valid JWT format with valid expiration
+    (SecureStore.getItemAsync as jest.Mock).mockImplementation((key: string) => {
+      if (key === 'accessToken') return Promise.resolve('mock-jwt-token');
+      if (key === 'userId') return Promise.resolve('user-123');
+      if (key === 'userEmail') return Promise.resolve('test@example.com');
+      if (key === 'legacyAccessToken') return Promise.resolve(mockToken);
+      if (key === 'legacyUserName') return Promise.resolve(mockUserName);
+      if (key === 'legacyUserId') return Promise.resolve('legacy-user-123');
+      return Promise.resolve(null);
+    });
+  });
+
+  describe('attachDeviceToBeneficiary', () => {
+    it('should handle deployment not found error', async () => {
+      // Mock getDeploymentForBeneficiary to return beneficiary without deployment
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            lastName: 'Garcia',
+            // No deploymentId
+          })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('DEPLOYMENT_NOT_FOUND');
+      expect(result.error?.message).toContain('Could not find beneficiary deployment');
+      expect(result.error?.status).toBe(404);
+    });
+
+    it('should handle missing authentication credentials', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        });
+
+      // Mock missing legacy credentials
+      (SecureStore.getItemAsync as jest.Mock).mockImplementation((key: string) => {
+        if (key === 'accessToken') return Promise.resolve('mock-jwt-token');
+        // Return null for legacy credentials
+        return Promise.resolve(null);
+      });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('UNAUTHORIZED');
+      expect(result.error?.message).toContain('Not authenticated');
+      expect(result.error?.status).toBe(401);
+    });
+
+    it('should handle 401 Unauthorized from Legacy API', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock 401 response from Legacy API
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 401,
+          json: async () => ({ message: 'Invalid credentials' })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('UNAUTHORIZED');
+      expect(result.error?.message).toMatch(/Authentication expired|Not authenticated/);
+      expect(result.error?.status).toBe(401);
+    });
+
+    it('should handle 404 Not Found from Legacy API', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock 404 response from Legacy API
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 404,
+          json: async () => ({ message: 'Device not found' })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('NOT_FOUND');
+      expect(result.error?.message).toContain('Sensor or deployment not found');
+      expect(result.error?.status).toBe(404);
+    });
+
+    it('should handle 500 Server Error from Legacy API', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock 500 response from Legacy API
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 500,
+          json: async () => ({ message: 'Internal server error' })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('SERVER_ERROR');
+      expect(result.error?.message).toContain('Server error');
+      expect(result.error?.status).toBe(500);
+    });
+
+    it('should handle 503 Service Unavailable from Legacy API', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock 503 response from Legacy API
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 503,
+          json: async () => ({ message: 'Service unavailable' })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('SERVICE_UNAVAILABLE');
+      expect(result.error?.message).toContain('Service unavailable');
+      expect(result.error?.status).toBe(503);
+    });
+
+    it('should handle Legacy API error status in response body', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock successful HTTP but error status in body
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          json: async () => ({
+            status: '400 Bad Request',
+            message: 'Invalid device MAC address format'
+          })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('LEGACY_API_ERROR');
+      expect(result.error?.message).toBe('Invalid device MAC address format');
+    });
+
+    it('should handle network errors', async () => {
+      // Mock successful beneficiary lookup first
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Then mock network error on Legacy API call
+        .mockRejectedValueOnce(
+          new Error('fetch failed')
+        );
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('NETWORK_ERROR');
+      expect(result.error?.message).toContain('No internet connection');
+    });
+
+    it('should handle unexpected exceptions', async () => {
+      // Mock successful beneficiary lookup first
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Then mock unexpected error
+        .mockRejectedValueOnce(
+          new Error('Unexpected error occurred')
+        );
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error?.code).toBe('EXCEPTION');
+      expect(result.error?.message).toContain('unexpected error');
+    });
+
+    it('should successfully attach device when all is well', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock successful device_form response
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          json: async () => ({
+            status: '200 OK',
+            message: 'Device attached successfully'
+          })
+        });
+
+      const result = await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      expect(result.ok).toBe(true);
+      expect(result.data).toEqual({ success: true });
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should send correct request parameters to Legacy API', async () => {
+      // Mock successful beneficiary lookup
+      (global.fetch as jest.Mock)
+        .mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            id: 123,
+            firstName: 'Maria',
+            deploymentId: mockDeploymentId
+          })
+        })
+        // Mock successful device_form response
+        .mockResolvedValueOnce({
+          ok: true,
+          status: 200,
+          json: async () => ({
+            status: '200 OK'
+          })
+        });
+
+      await api.attachDeviceToBeneficiary(
+        mockBeneficiaryId,
+        mockWellId,
+        mockDeviceMac
+      );
+
+      // Verify the second fetch call (device_form)
+      const secondCall = (global.fetch as jest.Mock).mock.calls[1];
+      expect(secondCall[0]).toBe('https://eluxnetworks.net/function/well-api/api');
+      expect(secondCall[1].method).toBe('POST');
+      expect(secondCall[1].headers['Content-Type']).toBe('application/x-www-form-urlencoded');
+
+      // Verify request body contains correct parameters
+      const bodyString = secondCall[1].body;
+      expect(bodyString).toContain('function=device_form');
+      expect(bodyString).toContain(`well_id=${mockWellId}`);
+      expect(bodyString).toContain(`device_mac=${mockDeviceMac.toUpperCase()}`);
+      expect(bodyString).toContain(`deployment_id=${mockDeploymentId}`);
+      expect(bodyString).toContain(`user_name=${mockUserName}`);
+      // Token will be URL encoded in the body, so just check it contains the token
+      expect(bodyString).toContain('token=');
+    });
+  });
+});
diff --git a/app/(tabs)/beneficiaries/[id]/setup-wifi.tsx b/app/(tabs)/beneficiaries/[id]/setup-wifi.tsx
index ef80c2d..dc80949 100644
--- a/app/(tabs)/beneficiaries/[id]/setup-wifi.tsx
+++ b/app/(tabs)/beneficiaries/[id]/setup-wifi.tsx
@@ -271,8 +271,9 @@ export default function SetupWiFiScreen() {
           mac
         );
         if (!attachResponse.ok) {
-          const errorDetail = attachResponse.error || 'Unknown API error';
-          throw new Error(`Failed to register sensor: ${errorDetail}`);
+          // Use the error message from the API response
+          const errorMessage = attachResponse.error?.message || 'Failed to register sensor';
+          throw new Error(errorMessage);
         }
       }
       updateSensorStep(deviceId, 'attach', 'completed');
diff --git a/services/api.ts b/services/api.ts
index 95be0d8..5051fd6 100644
--- a/services/api.ts
+++ b/services/api.ts
@@ -1935,19 +1935,33 @@ class ApiService {
     beneficiaryId: string,
     wellId: number,
     deviceMac: string
-  ) {
+  ): Promise<ApiResponse<{ success: true }>> {
     try {
       // Get deployment ID for beneficiary
       const deploymentResponse = await this.getDeploymentForBeneficiary(beneficiaryId);
       if (!deploymentResponse.ok) {
-        throw new Error(deploymentResponse.error.message);
+        return {
+          ok: false,
+          error: {
+            message: 'Could not find beneficiary deployment',
+            code: 'DEPLOYMENT_NOT_FOUND',
+            status: 404,
+          }
+        };
       }
 
       const deploymentId = deploymentResponse.data;
 
       const creds = await this.getLegacyCredentials();
       if (!creds) {
-        throw new Error('Not authenticated with Legacy API');
+        return {
+          ok: false,
+          error: {
+            message: 'Not authenticated. Please log in again.',
+            code: 'UNAUTHORIZED',
+            status: 401,
+          }
+        };
       }
 
       // Use device_form to attach device to deployment
@@ -1968,18 +1982,66 @@ class ApiService {
       });
 
       if (!attachResponse.ok) {
-        throw new Error(`Failed to attach device: HTTP ${attachResponse.status}`);
+        // Provide more specific error messages based on HTTP status
+        let errorMessage = 'Could not register sensor. Please try again.';
+        let errorCode = 'API_ERROR';
+
+        if (attachResponse.status === 401 || attachResponse.status === 403) {
+          errorMessage = 'Authentication expired. Please log in again.';
+          errorCode = 'UNAUTHORIZED';
+        } else if (attachResponse.status === 404) {
+          errorMessage = 'Sensor or deployment not found.';
+          errorCode = 'NOT_FOUND';
+        } else if (attachResponse.status === 500) {
+          errorMessage = 'Server error. Please try again later.';
+          errorCode = 'SERVER_ERROR';
+        } else if (attachResponse.status >= 500) {
+          errorMessage = 'Service unavailable. Please check your internet connection.';
+          errorCode = 'SERVICE_UNAVAILABLE';
+        }
+
+        return {
+          ok: false,
+          error: {
+            message: errorMessage,
+            code: errorCode,
+            status: attachResponse.status,
+          }
+        };
       }
 
       const data = await attachResponse.json();
 
       if (data.status !== '200 OK') {
-        throw new Error(data.message || `Legacy API error: ${data.status}`);
+        // Parse Legacy API error response
+        const errorMessage = data.message || 'Failed to register sensor';
+
+        return {
+          ok: false,
+          error: {
+            message: errorMessage,
+            code: 'LEGACY_API_ERROR',
+            status: attachResponse.status,
+          }
+        };
       }
 
-      return { ok: true };
+      return { ok: true, data: { success: true } };
     } catch (error: any) {
-      return { ok: false, error: error.message };
+      // Handle network errors and unexpected exceptions
+      const isNetworkError = error.message?.includes('network') ||
+                            error.message?.includes('fetch') ||
+                            (typeof navigator !== 'undefined' && !navigator.onLine);
+
+      return {
+        ok: false,
+        error: {
+          message: isNetworkError
+            ? 'No internet connection. Please check your network.'
+            : 'An unexpected error occurred. Please try again.',
+          code: isNetworkError ? 'NETWORK_ERROR' : 'EXCEPTION',
+        }
+      };
     }
   }