Today on the way to the office, the PCM threw some strange codes and entered reduced power mode. :( Turning the car off and on brought me back into normal mode, but still showing a check engine light.

1125 APP SYSTEM
1276 APP Sensor Circuit Performance
1278 TCS temporarily inhibited by PCM

What is the "APP SYSTEM" and how do I go about diagnosing the problem?

TIA.

 

APP is the Accelerator Pedal Position sensor mounted on the accelerator pedal assembly. There are three separate sensors within the housing, none of the sensors depends on the other sensors for operation. The sensors' circuits connect the APP to the Throttle Actuator Control (TAC) module.

The redundant APP sensors must agree on the position of the accelerator pedal for safe operation of the car. When the sensors disagree, this DTC is set.

A single APP sensor problem does not cause the control module to command the Reduced Engine Power mode. The control module uses the other two APP sensors to calculate throttle response.

A multiple APP sensor problem causes the control module to command the Reduced Engine Power Mode. There is limited throttle control.

When all three APP sensors are out of range, The control module commands the Reduced Engine Power Mode. The control module only allows the engine to idle.

P1125 is set when the APP sensors disagree, or out of range. P1276 is also set when the sensors disagree. I didn't find 1278 in my 2003 service manual (GM adds and deletes codes) TCS refers to the Traction Control System.

You might inspect the TAC module connectors for signs of water. The TAC module is located behind the passenger's side wheelhouse filler panel. That is behind the right front fender, between the wheelhouse and the dash panel, below the battery. It attaches to the Powertrain Control Module (PCM). If you remove the connector DO NOT touch any of the connector pins because of the potential for electrostatic discharge.

Access to a scan tool and service manual is a good idea to identify the problem.


[Modified by mapman, 10:34 AM 8/8/2003]

 

Thanks Mapman. Great post - very helpful.

This happened once before, about a month ago, but I ended up clearing the codes before I checked them. I'll delve into it this weekend.

 

Follow this TSB for updated TAC module :thumbs:

REDUCED ENG. POW. MESSAGE ALONG W/ PARTIC. PWRTRAIN #87-65-16 - (03/24/1998)
SUBJECT: REDUCED ENGINE POWER MESSAGE ALONG WITH PARTICULAR POWERTRAIN TROUBLE CODES (INSTALL NEW TAC-MODULE IF STRATEGY IN SERVICE MANUAL FAILS)

MODELS: 1997-98 CHEVROLET CORVETTE WITH 5.7L V8 ENGINE (VIN G - RPO LS1)

CONDITION #1: INTERMITTENT REDUCED ENGINE POWER EVENT, ACCOMPANIED BY P-1516, OR P1518 TROUBLE CODES.

IF A SITUATION ARISES WHERE A P-1516 TROUBLE CODE IS ACCOMPANIED BY A

P-1120 AND/OR P-1220 TROUBLE CODE, THIS SERVICE BULLETIN DOES NOT ----------------------------------------------------------------- APPLY. IN THESE CASES THE TECHNICIAN MUST USE THE NORMAL STRATEGY ------------------------------------------------------------------ BASED DIAGNOSTIC. -----------------

CONDITION #2: INTERMITTENT REDUCED ENGINE POWER ACCOMPANIED BY TROUBLE CODE P-1125 AND ANY OF THE FOLLOWING TROUBLE CODES:

P-1276 P-1281 P-1286

CAUSE:

CONDITION MAY BE DUE TO DIAGNOSTIC SOFTWARE AND CALIBRATION IN THE TAC-MODULE WERE NOT OPTIMALLY UTILIZED.

CORRECTION:

IF THE ABOVE CONDITIONS CANNOT BE CORRECTED USING THE NORMAL STRATEGY BASED DIAGNOSTICS LISTED IN THE SERVICE MANUAL, INSTALL THE NEW TAC- MODULE LISTED BELOW.

PART INFORMATION:

NEW TAC-MODULE PART # 25319443

PARTS ARE EXPECTED TO BE AVAILABLE ON APRIL 6, 1998 FROM GMSPO.

 

Thanks sooooo much for the TSB info. Probably saved me much work.

I tried the part number provided at GM Parts Direct, but no matches were found. Any idea on a price for the new TAC module? Is this something any dealership should be able to locate/price/order and sell over the counter?

 

Did you find the root cause of the problem?

 

Not specifically. It occurred to me that I don't go 100% throttle very often now that I am commuting. Not sure if it helped, but I tried cycling the throttle pedal through it's full range of motion 15 - 20 times.

No problem since then, but, so far, the problem has only happened twice, about 5 - 6 weeks apart. I need to dig through my shop manuals and see what I find based on the diagnostic procedure. I doubt taking it to the dealership will do much good until I can reliably reproduce the issue.