diff --git a/_cache_service_worker.js b/_cache_service_worker.js
index 36eeefb..bc3ddc4 100644
--- a/_cache_service_worker.js
+++ b/_cache_service_worker.js
@@ -14,7 +14,7 @@
 */
 
 var PRECACHE_ID='rom-patcher-js';
-var PRECACHE_VERSION='v261c';
+var PRECACHE_VERSION='v27';
 var PRECACHE_URLS=[
 	'/RomPatcher.js/','/RomPatcher.js/index.html',
 	'/RomPatcher.js/manifest.json',
diff --git a/index.html b/index.html
index 542e846..dfc4ab9 100644
--- a/index.html
+++ b/index.html
@@ -179,7 +179,7 @@
 		<button id="button-settings" class="button-outer"><img src="style/icon_settings.svg" class="icon settings" /> <span data-localize="settings">Settings</span></button>
 	</div>
 
-	Rom Patcher JS <small>v2.6.1</small> by <a href="/">Marc Robledo</a>
+	Rom Patcher JS <small>v2.7</small> by <a href="/">Marc Robledo</a>
 	<br />
 	<img src="style/icon_github.svg" class="icon github" /> <a href="https://github.com/marcrobledo/RomPatcher.js/" target="_blank">See on GitHub</a>
 	<img src="style/icon_heart.svg" class="icon heart" /> <a href="https://www.paypal.me/marcrobledo/5" target="_blank" rel="nofollow">Donate</a>
@@ -205,6 +205,7 @@
 					<option value="en">English</option>
 					<option value="fr">Français</option>
 					<option value="de">Deutsch</option>
+					<option value="it">Italiano</option>
 					<option value="es">Español</option>
 					<option value="nl">Nederlands</option>
 					<option value="sv">Svenska</option>
@@ -214,7 +215,6 @@
 					<option value="ru">Russian</option>
 					<option value="ja">日本語</option>
 					<option value="zh-cn">中文（简体）</option>
-					<option value="it">Italiano</option>
 				</select>
 			</div>
 		</div>
@@ -225,6 +225,11 @@
 		</div>
 
 		<div class="row m-b">
+			<div class="leftcol-lg"><label data-localize="fix_checksum">Fix ROM checksum</label></div>
+			<div class="rightcol-lg text-right"><span id="switch-fix-checksum" class="switch disabled"></span></div>
+		</div>
+
+		<div class="row">
 			<div class="leftcol-lg"><label data-localize="light_theme">Light theme</label></div>
 			<div class="rightcol-lg text-right"><span id="switch-theme" class="switch disabled"></span></div>
 		</div>
diff --git a/js/RomPatcher.js b/js/RomPatcher.js
index 3921246..2d5fbf5 100644
--- a/js/RomPatcher.js
+++ b/js/RomPatcher.js
@@ -1,4 +1,4 @@
-/* Rom Patcher JS v20220319 - Marc Robledo 2016-2022 - http://www.marcrobledo.com/license */
+/* Rom Patcher JS v20221110 - Marc Robledo 2016-2022 - http://www.marcrobledo.com/license */
 
 const TOO_BIG_ROM_SIZE=67108863;
 const HEADERS_INFO=[
@@ -246,6 +246,7 @@ var UI={
 var AppSettings={
 	langCode:(typeof navigator.userLanguage==='string')? navigator.userLanguage.substr(0,2) : 'en',
 	outputFileNameMatch:false,
+	fixChecksum:false,
 	lightTheme:false,
 
 	load:function(){
@@ -261,6 +262,10 @@ var AppSettings={
 					this.outputFileNameMatch=loadedSettings.outputFileNameMatch;
 					el('switch-output-name').className='switch enabled';
 				}
+				if(loadedSettings.fixChecksum===true){
+					this.fixChecksum=loadedSettings.fixChecksum;
+					el('switch-fix-checksum').className='switch enabled';
+				}
 				if(loadedSettings.lightTheme===true){
 					this.lightTheme=loadedSettings.lightTheme;
 					el('switch-theme').className='switch enabled';
@@ -429,6 +434,16 @@ addEvent(window,'load',function(){
 		}
 		AppSettings.save();
 	});
+	addEvent(el('switch-fix-checksum'), 'click', function(){
+		if(this.className==='switch enabled'){
+			this.className='switch disabled';
+			AppSettings.fixChecksum=false;
+		}else{
+			this.className='switch enabled';
+			AppSettings.fixChecksum=true;
+		}
+		AppSettings.save();
+	});
 	addEvent(el('switch-theme'), 'click', function(){
 		if(this.className==='switch enabled'){
 			this.className='switch disabled';
@@ -557,6 +572,82 @@ function validateSource(){
 
 
 
+
+
+function _getRomSystem(file){
+	if(file.fileSize>0x0200 && file.fileSize%4===0){
+		if(/\.gbc?/i.test(file.fileName)){
+			var NINTENDO_LOGO=[
+				0xce, 0xed, 0x66, 0x66, 0xcc, 0x0d, 0x00, 0x0b, 0x03, 0x73, 0x00, 0x83, 0x00, 0x0c, 0x00, 0x0d,
+				0x00, 0x08, 0x11, 0x1f, 0x88, 0x89, 0x00, 0x0e, 0xdc, 0xcc, 0x6e, 0xe6, 0xdd, 0xdd, 0xd9, 0x99
+			];
+			file.offset=0x104;
+			var valid=true;
+			for(var i=0; i<NINTENDO_LOGO.length && valid; i++){
+				if(NINTENDO_LOGO[i]!==file.readU8())
+					valid=false;
+			}
+			if(valid)
+				return 'gb';
+		}else if(/\.(bin|md)?/i.test(file.fileName)){
+			file.offset=0x0100;
+			if(/SEGA (GENESIS|MEGA DR)/.test(file.readString(12)))
+				return 'smd';
+		}
+	}
+	return null;
+}
+function _getHeaderChecksumInfo(file){
+	var info={
+		system:_getRomSystem(file),
+		current:null,
+		calculated:null,
+		fix:null
+	}
+	
+	if(info.system==='gb'){
+		/* get current checksum */
+		file.offset=0x014d;
+		info.current=file.readU8();
+
+		/* calculate checksum */
+		info.calculated=0x00;
+		file.offset=0x0134;
+		for(var i=0; i<=0x18; i++){
+			info.calculated=((info.calculated - file.readU8() - 1) >>> 0) & 0xff;
+		}
+
+		/* fix checksum */
+		info.fix=function(file){
+			file.offset=0x014d;
+			file.writeU8(this.calculated);
+		}
+
+	}else if(info.system==='smd'){
+		/* get current checksum */
+		file.offset=0x018e;
+		info.current=file.readU16();
+
+		/* calculate checksum */
+		info.calculated=0x0000;
+		file.offset=0x0200;
+		while(!file.isEOF()){
+			info.calculated=((info.calculated + file.readU16()) >>> 0) & 0xffff;
+		}
+
+		/* fix checksum */
+		info.fix=function(file){
+			file.offset=0x018e;
+			file.writeU16(this.calculated);
+		}
+	}else{
+		info=null;
+	}
+
+	return info;
+}
+
+
 function _readPatchFile(){
 	setTabApplyEnabled(false);
 	patchFile.littleEndian=false;
@@ -622,18 +713,18 @@ function preparePatchedRom(originalRom, patchedRom, headerSize){
 
 
 	/* fix checksum if needed */
-	//var fixedChecksum=fixConsoleChecksum(patchedRom);
+	if(AppSettings.fixChecksum){
+		var checksumInfo=_getHeaderChecksumInfo(patchedRom);
+		if(checksumInfo && checksumInfo.current!==checksumInfo.calculated && confirm(_('fix_checksum_prompt')+' ('+padZeroes(checksumInfo.current)+' -> '+padZeroes(checksumInfo.calculated)+')')){
+			checksumInfo.fix(patchedRom);
+		}
+	}
 
 
 
 
 	setMessage('apply');
 	patchedRom.save();
-
-
-	/*if(fixedChecksum){
-		setMessage('apply','Checksum was fixed','warning');
-	}*/
 	
 	//debug: create unheadered patch
 	/*if(headerSize && el('checkbox-addheader').checked){
@@ -642,6 +733,8 @@ function preparePatchedRom(originalRom, patchedRom, headerSize){
 }
 
 
+
+
 /*function removeHeader(romFile){
 	//r._dataView=new DataView(r._dataView.buffer, headerSize);
 	oldHeader=romFile.slice(0,headerSize);
diff --git a/js/locale.js b/js/locale.js
index 9ecc6fe..f336221 100644
--- a/js/locale.js
+++ b/js/locale.js
@@ -3,6 +3,7 @@ const LOCALIZATION={
 		'creator_mode':				'Creator mode',
 		'settings':					'Settings',
 		'alternate_output_name':	'Use patch name for output',
+		'fix_checksum':				'Fix ROM header checksum',
 		'light_theme':				'Light theme',
 
 		'apply_patch':				'Apply patch',
@@ -14,6 +15,7 @@ const LOCALIZATION={
 		'applying_patch':			'Applying patch...',
 		'downloading':				'Downloading...',
 		'unzipping':				'Unzipping...',
+		'fix_checksum_prompt':		'Fix ROM header checksum?',
 
 		'create_patch':				'Create patch',
 		'original_rom':				'Original ROM:',
@@ -33,6 +35,7 @@ const LOCALIZATION={
 		'creator_mode':				'Modo creador',
 		'settings':					'Configuración',
 		'alternate_output_name':	'Guardar con nombre del parche',
+		'fix_checksum':				'Corregir checksum cabecera ROM',
 		'light_theme':				'Tema claro',
 
 		'apply_patch':				'Aplicar parche',
@@ -44,6 +47,7 @@ const LOCALIZATION={
 		'applying_patch':			'Aplicando parche...',
 		'downloading':				'Descargando...',
 		'unzipping':				'Descomprimiendo...',
+		'fix_checksum_prompt':		'¿Corregir checksum en la cabecera de la ROM?',
 
 		'create_patch':				'Crear parche',
 		'original_rom':				'ROM original:',