My 2003 Prius is suffering from the cold-weather start problem (I purchased it used last Spring). Whenever the temps drop below about 50 degrees, it won't start. It is very consistent in this symptom and I've replaced the 12v battery and cleaned out the oil in the manifold as recommended in the forum to no avail. The other key suggestion in the forum is to replace the ECU with the latest 89661–47054 model. However, looking at TSB EG011-03, my ECU may already be the latest model based upon the VIN. Is there a way to read the ECU part number (is it accessible enough to read without removal)?
The engine ECU is located behind the glove box area, so you might remove the glove box to see whether the part number label is visible. Since your model year is 2003 there is a good chance the latest ECU revision is installed. If you haven't already done so, you should clean the throttle body interior and throttle plate. That may help the cold weather no-start issue. The correct procedure for that is to remove the air cleaner housing, apply throttle body cleaner to a clean cloth or paper towel, then use that to clean the throttle body interior and throttle plate. Rotate the spring-loaded throttle shaft for better access to the throttle plate. Also, check the circuit opening relay (aka fuel pump relay) for proper operation, which is located within the relay box near the inverter. (Apply 12VDC to the coil terminals and measure resistance using a digital multimeter, across the switched terminals. If more than 0.5 ohms, replace the relay.) If the relay switched contacts have significant resistance, that will reduce the voltage applied to the fuel pump which will result in suboptimal fuel line pressure and engine performance. Finally, make sure that the engine oil viscosity is no heavier than 5W-30. You may want to try 0W-30 during the winter season.
Thanks Patrick. The part number isn't quite visible, so looks like I'll need to remove it. I did clean out the throttle body interior a few weeks ago, which seemed help (e.g. it needs to be colder out before it stops working), but it didn't resolve it. I haven't tested the fuel pump relay so I'll give that a try too. I'll replace the oil as well. Thanks.
I take that back, I can see the part number and it is indeed the newer 47054 ECU. I'll check the relay. Cheers.
If you don't find a relay problem, you might also consider replacing the iridium spark plugs with the correct NGK or Denso plugs, if they have logged more than 60 K miles (which is the Classic service interval.)
