Throttle/accelerator Position Sensor Problem (p1120/p1121)

[smaus]

There are several threads here that report about problems with a lack of acceleration when the vehicle is cold (sub-freezing). Using a OBD scan tool I could see that this condition produces error codes p1121 and p1120 in the ECU.

Following the recommendations from previous postings, I first cleaned the mass air flow sensor. This had no effect. The readings from the mass flow sensor are 5 g/s (0.7 lbs/min) at idle and 30 g/s (4 lbs/min) at 4000 RPM. Seems pretty low. Anyone know the normal range for this?

I then found that when I heat the throttle body (on the side where the accelerator cable is attached to the accelerator pedal position sensor) with a hair dryer before starting the car, the problem does not occur. :)

A really neat tool that one can get is a scan tool with USB cable, which can be attached to a laptop (about $60 at Amazon). Using free software one can then monitor the sensor outputs. Really cool.

However, there are two sensors on the throttle body: a throttle position sensor ($140 on Amazon) and an accelerator pedal position sensor ($299, only available at Toyota or Lexus). Unfortunately, the scan tool output is only for the "absolute throttle position", and it is not clear which of the two are monitored (The name "absolute throttle position" seems to be the same with all software).

In my case, the "absolute throttle position" output at warm temperature is: 15% for accelerator pedal released, 80% for pedal pressed down, while at cold temperature it is 17% for pedal released and 21% for pedal pressed down. Seems like a clear-cut case of a defective sensor.

The problem is only: Can I be sure that this is due to a defective accelerator pedal position sensor (PPS)? Does anyone know, whether the "absolute throttle position" monitors the PPS or the throttle position sensor (TPS)? If it is the TPS, could it be a defective servo motor driving the throttle opening, rather than a defective TPS?

Thanks for any advice,

Stefan

1998 LX470