Hey guys,

Getting your opinions to see if I've conclusively proved it's the throttle position sensor I need to replace in my 1997 C5.

So I'm getting Reduced Engine Power of course, car really labors under 2k rpms, idling irregularly.

Codes that are current:
** P1221
throttle position sensor 1-2 correlation
** C2120
TPM System Malfunction

(I'll assume the tire pressure monitor system is not relevant here. Just wanting to fix the engine problem!)

So I took a digital multimeter (dmm) to the tire pressure sensor connector.

I verified it was measuring correctly by measuring the battery at idle (~11.8V)



Then I measured the pink wire which is sensor 2 and the dark blue wire which is sensor 1.

I disconnected the harness from the throttle body to directly measure the output of sensor 1 and sensor 2.

Fortunately my car doesn't always throw reduced engine.

So the measuring session started without the reduced engine power state, and I was able to measure voltage would go up and down alternately for sensors 1 and 2.
I did this by

1) disconnecting the harness from the throttle body
2) inserting my dmm probe in the correct harness lead position (from the factory service manual) then having a friend alternate between no throttle, half way throttle, full throttle.


Measuring Sensor #1 at the 4th position in the harness

Measuring Sensor #2 at the first position in the harness


When reduced engine power was NOT showing up,
Sensor 1 correctly decreased from 4V down to ~0V as the throttle was pressed.
Sensor 2 conversely increased from 0.3V to 4V(?) as the throttle was pressed.

Luckily right after I started measure the reduced engine power state started!
At that point,
Sensor 1 stayed at 4V.
Sensor 2 stayed at 0.3V.
This remained the voltage no matter if the throttle was pressed or not.

Is this definitive proof that I need to change the Throttle Position Sensor? Any other things to check?

It's definitely weird that the measuring goes from successful to total failure seemingly with no gating event.
The car just flips out! 🤔

OR do I need to figure out what's causing the TPS to go from working to non-working? Will sensors work then not work? Sporadically?
I had my battery and alternator tested, both by autozone and by a battery tester I have, battery was good, alternator was good.

Thanks for any ideas!

 

Oh sorry guys theres a whole procedure in the factory service manual i need to follow to diagnose. I may return after following said procedure thanks!

 

I have no idea by the pictures what you are checking but there are 4 voltages you should be checking at the TPS...TPS1 at idle and at WOT key on engine off (KOEO) and the same for TPS2...you do this by backprobing both signal wires with the connector installed...with connector unplugged you should have a 5 volt ref. on both signal wires...there should be less than 100mv on the sensor grounds..TPS1 voltage increases as the throttle is depressed from idle to WOT...TPS2 works the opposite way...these are the 4 voltages from my TPS but I don’t have them marked. Another thing is those signal wires should really be checked with a scope..a DMM is too slow to pick up a glitch or dropout in the voltage...any dropout in the voltage will trigger the DTC !!

 

Thanks for all the advice and time you took to write that up!

Haven't had a chance to try it again but I will take your advice thanks!