2004 Aveo, Automatic, P0452, P0122, Reference voltages low, stumped!

Hi Everyone,

I have a 2004 Aveo, Automatic. I'm hoping someone can help me because I'm out of ideas. I was driving down the road recently (runs fine, 60k miles) and the Check Engine Light came on and it's throwing P0452 and P0122 (and P0700). It won't shift into 4th gear nor lock up. It shifts hard when putting it into Drive or Park or Reverse.

P0122 Is the Throttle Position Sensor, and the ECM is not supplying the 5v reference voltage. Instead the reference voltage is about 0.23v. Checked and replaced wiring, measured resistance, etc. The service manual says the ECM should be replaced. Tried that, still throwing same codes, still not supplying the correct 5v.

P0452 is the Evap System Pressure (Tank Pressure) low voltage. Same problem. Not supplying 5v reference.

Checking more of the service manual it says the Transmission Control Module (TCM) should be supplying 12v on one of the circuts to the ECM, but it's only supplying 2.6v instead of 12! So I replaced the TCM, and THAT didn't make a difference.

Why the heck are these computers not providing the correct reference voltage even when replaced?!

The TPS is simple : 5v, one ground, which goes through the TPS resistor and returns a value less than 5v. Yet my ECM, and replacement ECM isn't showing 5v. The scan tool even shows the pedal moving, just no where near as far as it's supposed to at 100%.

I'm totally stumped. This car is in great shape and only 60k miles and I'm ready to part it out because of these computer problems.

Lastly, when I clear the codes, the P0122 and P0452 will not clear. Both codes set again immediately after clearing. I don't even need to start the engine to set these codes.

Any ideas?????????

This is rabbit's thing, but I have to ask if you have changed the timing belt yet?

the codes reset immediately because the problem is still there.

did you verify the ecu grounds?

The timing belt has not jumped or broken. The engine runs perfectly. It is about due for a timing belt which is next on my list if I can figure this problem out first.

Rabbit: You are correct, the codes reset because those 2 sensors are not getting the required 5v. I did check and re-ground the ECU grounds to make sure they were getting a good connection. It did not solve anything.

Both the TPS and the Evap sensors are only reading 0.23v from the 5v supply wire.

I can actually get rid of the TPS code if I hold the gas pedal down the entire time (when erasing the code). But as soon as I let go it sets that code again.

According to my service manuals, the TCM should have 12v on the serial connections to the ECM. These are the ones reading only 2.6v. I've checked the resistance on all of these wires and everything checks out fine.

Is there a ground or a supply wire somewhere before the TCM? I don't know what to check next. The service manual doesn't give me any more options to check. It just says replace the TCM and ECM, which I've already tried.

FINALLY I found the problem. Apparently the A/C Refridgerant Pressure switch is shorted out. The A/C Pressure Circuit runs on the same exact circuit as the Fuel Tank Pressure Sensor. So when I unplugged the A/C Sensor the 5v reference voltage came right back. Unfortunately replacing this sensor will probably require a professional since I'm sure the A/C needs discharged and recharged to replace it.

It would be nice if these all used separate circuits so it didn't send me on a wild goose chase for a month... what a pain in the neck!

that would do it, the eco does only have one circuit for the 5v reference. glad you found it.

The service manual doesn't state anything about having to discharge/recharge the A/C when changing this pressure switch. That doesn't seem right. Does anyone know how easy it is to replace the Air Conditioning (A/C) Refrigerant Pressure Sensor?