2001 Subaru Outback P0122 code

[jemcdon]

Hi,

 

I have a 2001 subaru outback, 2.5L, manual base model with 101K miles on it.    The car has been  throwing a P0122 code persistently for a while now.    At first, the code was rarely thrown (about a year ago) and only just after starting up the car.   I have not had an occassion where the code was thrown outside of the initial cold start.    The code will be thrown within the first minute of the car starting.    

 

This code corresponds to the TPS circuit and I have had the car in the shop where they replace the sensor wire to the PCM.     At that time, the P0122 was less frequently thrown, perhaps once a week.   For the past few weeks, the P0122 is thrown at 9 out of 10 cold starts.   The mechanic who worked on it before suggested that the problem may be with the PCM and not with the sensor.  

 

I checked the ground cable and the power input to the TPS last week.   The ground seemed OK, once the switch was turned on, the TPS header showed that there was  0 ohm to ground and 0V.  The voltage input to the TPS was close to 5V.  

 

Questions to the board:

0) Is the TPS a red herring for some other issue? 

1) Is it more likely the throttle position sensor is bad or the PCM has a bad input for the TPS?  

2) If I replace the PCM in this model, assuming I match up the same part number, would I need to do any programming of the PCM?  I feel confident that I could replace it other wise.   

 

Thanks,

Jeff

[Fairtax4me]

5 volts on the reference wire is good. Ground should show close to 0 ohms with the key on. The third pin should show around 0.5v with the throttle fully closed. Use a paper clip to back-probe the connector while its still plugged in.

If the voltage is less than 0.45v the TPS will need to be adjusted.

If the voltage is above 0.55v the throttle bore may need to be cleaned. Buildup in the throttle bore and on the edge of the throttle plate can hold the throttle plate slightly "open" and cause the TPS signal to be out of range when the throttle is closed.

 

The problem may be that someone has tried to adjust idle speed by moving the throttle stop screw on the side of the throttle body. There should be paint marks on the screw and the locking nut, check to see of they align, or look like they've been disturbed.

If the throttle stop screw has been disturbed you'll need to verify the clearance between the throttle plate and bore inside the throttle body. The throttle plate should stop just before hitting the bore of the throttle body. Adjust the stop screw until it stops the throttle plate just before the plate hits the bore.

Now you'll need to adjust the TPS sensor by loosening the holding screws. Adjust until the sensor output pin shows 0.5v. Once adjusted, reset the ECU by unhooking the battery negative for a few minutes. Reconnect then start the engine and allow it to run at idle to operating temp then go for a drive.

 

If the codes persists after cleaning the throttle body and adjusting the sensor, the sensor may need to be replaced.