Unfortunately on the Yaris it is a common problem. Toyota has a warranty for 36 months / 36,000 miles for this problem, but if the car out of the warranty you need to pay for the repair.
The good thing is that the harness can be replaced in less than 2 hours.
Here is a TSB which tells a bit more:
Electrical - MIL ON/Multiple DTC's Set
November 5, 2008
M.I.L. "ON" DTC P010#, P0121, P0125, P0172, P035#, P2237, or P2238
Some 2007 - 2008 model year Yaris vehicles may exhibit a M.I.L. "ON" DTC condition with one of the following Diagnostic Trouble Codes (DTCs):
- P0101 - Mass Air Flow Circuit Range / Performance Problem
- P0102 - Mass Air Flow Circuit Low Input
- P0103 - Mass Air Flow Circuit High Input
- P0121 - Throttle / Pedal Position Sensor/Switch "A" Circuit Range/Performance Problem
- P0125 - Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0172 - System Too Rich
- P035# - Ignition Coil "#" Primary / Secondary Circuit
- P2237 - Oxygen (A/F) Sensor Pumping Current Circuit / Open
- P2238 - Oxygen (A/F) Sensor Pumping Current Circuit Low
An improved engine wire harness has been developed to address this condition. Use the following repair procedure to diagnose and repair customer complaint vehicles.
Roy, on alldata you can subscribe to receive new TSBs. They will e-mail them to you.
There are so many from different manufacturers, its impossible to keep all in mind, I usually read them and later it could give a clue.
I just left ma yaris today , code PO121 , 2months after warranty (34000km).They told me after diagnostics that I need a new harness (700 plus 350 labour) .Told them that i consider it ridiculous after such mileage,complaint went to Toyota,will see what happens tomorrow .
Well, in the recall info above Toyota included the time allowance -among the part numbers and VIN# info - and it's 1.8 hours. Unfortunately it's in a graphic (picture?) format and I can't post it here, but if you post an e-mail I can send you the entire thing.