diff --git a/lib/census_api.rb b/lib/census_api.rb
index 7078fa828..0eb3ee35d 100644
--- a/lib/census_api.rb
+++ b/lib/census_api.rb
@@ -1,7 +1,54 @@
 class CensusApi
 
   def call(document_type, document_number)
-    Response.new(get_response_body(document_type, document_number))
+    response = nil
+    get_document_number_variants(document_type, document_number).each do |variant|
+      response = Response.new(get_response_body(document_type, variant))
+      return response if response.valid?
+    end
+    response
+  end
+
+  def get_document_number_variants(document_type, document_number)
+    # Delete all non-alphanumerics
+    document_number = document_number.to_s.gsub(/[^0-9A-Za-z]/i, '')
+    variants = []
+
+    if is_dni?(document_type)
+      # If a letter exists at the end, delete it and put it on the letter var
+      letter = document_number.last
+      if letter[/[A-Za-z]/] == letter
+        document_number = document_number[0..-2]
+      else
+        letter = nil
+      end
+
+      document_number = document_number.last(8) # Keep only the last 7 digits
+      document_number = document_number.gsub(/^0+/, '') # Removes leading zeros
+
+      # if the number has less than 7 digits, pad with zeros to the left and add each variant to the list
+      # For example, if the initial document_number is 1234, possible numbers should have
+      # [1234, 01234, 001234, 0001234]
+      possible_numbers = []
+      possible_numbers << document_number unless document_number.blank?
+      while document_number.size < 8
+        document_number = "0#{document_number}"
+        possible_numbers << document_number
+      end
+
+      variants += possible_numbers
+
+      # if a letter was given, try the numbers followed by the letter in upper and lowercase
+      if letter.present? then
+        possible_numbers.each do |number|
+          variants << number + letter.downcase << number + letter.upcase
+        end
+      end
+    else # not a DNI
+      variants << document_number
+    end
+
+    variants
   end
 
   class Response
@@ -60,4 +107,8 @@ class CensusApi
     def stubbed_response_body
       {:get_habita_datos_response=>{:get_habita_datos_return=>{:hay_errores=>false, :datos_habitante=>{:item=>{:fecha_nacimiento_string=>"31-12-1980", :identificador_documento=>"12345678Z", }}, :datos_vivienda=>{:item=>{:codigo_postal=>"28013"}}}}}
     end
+
+    def is_dni?(document_type)
+      document_type.to_s == "1"
+    end
 end
diff --git a/spec/lib/census_api_spec.rb b/spec/lib/census_api_spec.rb
new file mode 100644
index 000000000..a49133d23
--- /dev/null
+++ b/spec/lib/census_api_spec.rb
@@ -0,0 +1,55 @@
+require 'rails_helper'
+
+describe CensusApi do
+  let(:api) { described_class.new }
+
+  describe '#get_document_number_variants' do
+    it "trims and cleans up entry" do
+      expect(api.get_document_number_variants(2, '  1 2@ 34')).to eq(['1234'])
+    end
+
+    it "returns only one try for passports & residence cards" do
+      expect(api.get_document_number_variants(2, '1234')).to eq(['1234'])
+      expect(api.get_document_number_variants(3, '1234')).to eq(['1234'])
+    end
+
+    it 'takes only the last 8 digits for dnis and resicence cards' do
+      expect(api.get_document_number_variants(1, '543212345678')).to eq(['12345678'])
+    end
+
+    it 'tries all the dni variants padding with zeroes' do
+      expect(api.get_document_number_variants(1, '0123456')).to eq(['123456', '0123456', '00123456'])
+      expect(api.get_document_number_variants(1, '00123456')).to eq(['123456', '0123456', '00123456'])
+    end
+
+    it 'adds upper and lowercase letter when the letter is present' do
+      expect(api.get_document_number_variants(1, '1234567A')).to eq(['1234567', '01234567', '1234567a', '1234567A', '01234567a', '01234567A'])
+    end
+  end
+
+  describe '#call' do
+    let(:invalid_body) { {get_habita_datos_response: {get_habita_datos_return: {datos_habitante: {}}}} }
+    let(:valid_body){ {get_habita_datos_response: {get_habita_datos_return: {datos_habitante: {item: {fecha_nacimiento_string: "1/1/1980"}}}}} }
+
+    it "returns the response for the first valid variant" do
+      allow(api).to receive(:get_response_body).with(1, "00123456").and_return(invalid_body)
+      allow(api).to receive(:get_response_body).with(1, "123456").and_return(invalid_body)
+      expect(api).to receive(:get_response_body).with(1, "0123456").and_return(valid_body)
+
+      response = api.call(1, "123456")
+
+      expect(response).to be_valid
+      expect(response.date_of_birth).to eq('1/1/1980')
+    end
+
+    it "returns the last failed response" do
+      expect(api).to receive(:get_response_body).with(1, "00123456").and_return(invalid_body)
+      expect(api).to receive(:get_response_body).with(1, "123456").and_return(invalid_body)
+      expect(api).to receive(:get_response_body).with(1, "0123456").and_return(invalid_body)
+      response = api.call(1, "123456")
+
+      expect(response).to_not be_valid
+    end
+  end
+
+end