Free app with full custom PID support, no hidden fees: https://itunes.apple.com/us/app/engine-link-le/id1222128102?ls=1&mt=8
LELink new version is an engine performance and diagnostics tool for both automotive enthusiasts and professionals with better performance. Connecting to your iPhone/iPod, this scanner allows you to easily scan and clear engine codes, view and save real-time engine and performance data and much more.
Bluetooth Low Energy
Low energy footprint and longer battery time
Fast refresh rate
Automatic configure / connect
Plug and Play. NO Pairing is needed

Requires:
iPhone 4s or later
iPad 3rd gen or later
iPad mini or later
iPod touch 5th gen or later
Supported Apps:
Engine Link - OBD II vehicle monitoring and diagnosis
OBD Fusion - OBD2 vehicle scan tool and diagnostics
ezOBD
LeafSpy Pro
Mode6 OBD Scanner
Dyno Chart - OBD II Engine Performance Tool

EngineLink features:
+ Display vehicle live data with full customizable gauge set
+ Show and reset a DTC / CEL / fault code like a scan tool
+ Graph data + MPG widget to plot real time fuel efficiency
+ Heads up display / HUD mode for night time driving
+ Read Transmission Temperatures (vehicle dependent)
+ Horsepower (HP), Torque, and acceleration
+ Boost / vacuum gauge
+ Engine speed / Engine RPM / Engine Load / Coolant / Fuel Tank
+ Full configurable GUI with user choice of dial, bar, plot, digital, and on-off gauge as display elements
+ Formula support
+ Preloaded GM and Ford PID, standard PID, Prius PID libraries and more
+ User can add more libraries within the app or by using iTunes File Sharing
+ Background color option
+ Email user log function
+ Multiple PID formula support
+ 0 to 60mph timer and quarter mile timer