Service Guys Clueless on error P0128 - Toyota Yaris Forums

CoryM · 06-23-2015, 11:51 PM

P0128 means the computer is seeing coolant temps below the minimum (whether it is or not). 99% of the time this means the thermostat is stuck open, and the engine warms up too slowly. Tsats sometimes act up intermittently, so I don't blame them for starting with a Tsat even if it tested out OK in the shop. It's step 1 for sure. Since it seems the Tstat is not the problem, now it gets a little harder (especially since it isn't happening all the time). It could be: faulty sensor/wiring, faulty ECU, faulty ECU software, low coolant (leak), incorrectly mixed coolant, waterpump not circulating and a few other things. I know it's frustrating as a customer, but unless the problem happens quickly so you can test it while it's acting up, it can be very tough to diagnose.

I am not sure what "cord" they are mentioning. If you could get a better description, I may be able to advise further. $1200 goes a long way on a Yaris so I am curious what they are wanting to do. Possibly a wiring harness?

Cheers.

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06-23-2015, 11:51 PM	#2
CoryM Drives: 2009 5-door, 5-speed Join Date: Sep 2011 Location: White Rock BC Posts: 687	P0128 means the computer is seeing coolant temps below the minimum (whether it is or not). 99% of the time this means the thermostat is stuck open, and the engine warms up too slowly. Tsats sometimes act up intermittently, so I don't blame them for starting with a Tsat even if it tested out OK in the shop. It's step 1 for sure. Since it seems the Tstat is not the problem, now it gets a little harder (especially since it isn't happening all the time). It could be: faulty sensor/wiring, faulty ECU, faulty ECU software, low coolant (leak), incorrectly mixed coolant, waterpump not circulating and a few other things. I know it's frustrating as a customer, but unless the problem happens quickly so you can test it while it's acting up, it can be very tough to diagnose. I am not sure what "cord" they are mentioning. If you could get a better description, I may be able to advise further. $1200 goes a long way on a Yaris so I am curious what they are wanting to do. Possibly a wiring harness? Cheers.