How to Diagnose

[99StSBoy]

I was wondering if anyone could tell me what I need in order to diagnose the codes on my 99 STS. The check engine light recently came on and seems to come and go. The car runs perfectly fine but this light is buggin me. Is there something that the car has to assist me in this or do I need a diagnostic tool? Any help would be greatly appreciated.

[Bruce Nunnally]

Your Cadillac has onboard diagnostics that can be displayed in the driver's information center. Check http://caddyinfo.netgetgoing.com/readingcodes.html for how to display for various years. Then post the codes you get here.

[99StSBoy]

Thanks Bruce, I really appreciate it. Ok, the codes I got are as follows...
AMP U1064 History
PCM P0404 History
PCM P1522 History
PCM P1810 History


I did get the codes while the car was running also...

I don't know what they mean and hope that maybe you guys could shed some light.... Hate to make my first posts this way, thanks in advance. BTW it is a 99 STS.

I luv my car, hate that light

[Bruce Nunnally]

DTC U1064 Loss of Communications with DIM
(DIM = Dash Integration Module)
The class 2 serial data line communicates information between modules. Each module connected to the serial data line is assigned a recognition code, or address. This code is used in order to identify which modules are communicating. Modules communicate with each other by periodically sending state of health (SOH) messages. When the ignition switch is in RUN, each module communicating on the serial data line sends a SOH message every 2 seconds. This ensures that the system is operating properly. When the dash integration module (DIM) stops communicating on the class 2 serial data line, the SOH that is normally sent on the serial data line every 2 seconds disappears. The module, which expects to receive that message, detects the absence of the message. The module in turn sets DTC U1064 associated with the loss of SOH of the DIM

DTC P0404 Exhaust Gas Recirculation (EGR) Open Position Performance (bingo - this will set a light for you)
This diagnostic test checks for a malfunction in the circuitry and hardware of the Linear EGR valve. The ability of the PCM to control the EGR valve is checked with this test. When the PCM commands the EGR valve to a desired position and changes that position less than a calibratable amount, this test then checks the EGR pintle position error. If the pintle position error, which is the difference between the desired pintle position and the actual pintle position, is too great then this DTC is set.

My reference does not show P1522. Here are some close to that:
DTC P1520 Transmission Range Switch Circuit
DTC P1524 Throttle Closed Position Performance
DTC P1527 Transmission Range to Pressure Switch Correlation

DTC P1810 TFP Valve Position Switch Circuit
The automatic transmission fluid pressure (TFP) manual valve position switch consists of five normally open switches. Ignition voltage is applied to each of the switches. By grounding one or more of these switches with fluid pressure from the manual valve, the PCM can detect which gear range is selected.
If the PCM detects an invalid combination of the TFP manual valve position switch, then DTC P1810 sets. DTC P1810 is a Type B DTC.

So looks like you need to clean or replace your EGR valve, and you are having some type of transmission switching issue.

[99StSBoy]

wow, thanks a lot bruce. I really appreciate the information. Is the egr hard to change out. I am handy with tools but some of this stuff on the car I am unfamilar with. I like working on cars and since my warranty is just out I would rather do this myself than pay the dealer high dollars to fix something that may take only a few hours or so. I think I will do some research and get me a manual... Thanks for the help and information.

[Bruce Nunnally]

Here is more info on EGR valve cleaning:
http://caddyinfo.netgetgoing.com/how...vecleaning.htm

Check ebay or helminc.com for a shop manual; they are great.

Check gmotors.com, gmpartsdirect.com, or parts.com for discount new parts.