p1634 & u1255

[1997 sts]

how do i fix codes p1634 and u1255 on my 1997 sts?

[Cadillac Jim]

P1634 Ignition 1 Switch Circuit (Ignition 1 power low voltage)

U1255 Class 2 Communication Malfunction (Serial Data Line Malfunction)

The U1255 can be thrown by just about any module, but most often it is thrown by the IPC module. If it is a HISTORY code and there are no other codes that could be related to it, you can probably ignore it. When we are done with the P1634 code you can reset the codes and see if it comes back.

I'm going by the 1997 FSM page 6-602, the section on P1634. If the MIL (Check Engine) light is on, either the code is CURRENT or was thrown in the last two times you started the car. The P1634 code is thrown when one of the three power sources for the PCM is disrupted. The first thing to check are the IGN 0 and PCM (IGN) 10 Amp fuses in the engine compartment fuse block. If either is blown (or cracked) the PCM will throw the P1634 code. The two voltages that the PCM checks come in on the LT GRN wire to terminal 18 and the PNK wire on terminal 19, both on connector C1. Both come through the ignition switch. The LT GRN wire is hot when the ignition switch is in OFF and RUN but not in BULB TEST or START. The PNK wire is hot in RUN, BULB TEST and START.

You didn't mention any other codes, or whether you were having other problems such as the engine not shutting off when you turn off the key. If P0560, B1327, or any other system voltage code was set, the battery voltage dropped below 9 Volts during starting and you have a low battery, a poor connection on one of the battery cables, or a failing battery, and you need to get the battery and battery cables in good shape, reset the codes, and see if any of them come back.

You didn't say whether the P1634 code was CURRENT or HISTORY. If it is CURRENT and stays that way, likely one of the two fuses that I mentioned is blown. You can tell which by turning the key on (it's not necessary to start the engine) and seeing if both the LT GRN wire and the PNK wire are hot, and have the same voltage (12 Volts to 12.6 Volts with the engine off). If it is HISTORY, it could still be one of the fuses but it would be cracked, not blown, and might check good or bad depending on its' mood.

There is a possibility that your ignition switch is starting to go bad. If you have a HISTORY code, clear it, and it comes back within a week or so, you have an intermittent problem that could easily be the ignition switch. The PCM stores "freeze-frame" data that the dealer can read using a Tech II, or other mechanics can read using high-end scan tools, and that can tell you which of the two voltages is low.

The FSM says that if the code is HISTORY and the problem is intermittent, check a splice in the PNK wire between the fuse block and the PCM where it adds a PNK wire to provide two contacts for the PCM.

[1997 sts]

thanks for the help! i really need to get it smogged(nevada), will that code keep me from passing if its lighting up the check engine.

[BodybyFisher]

Not only a bad fuse/breaker but a bad connection from corrosion white or green on breakers supplying power to the PCM or a bad battery, follow Jim's advice above

[1997 sts]

i checked fuses and cleared p1634 code but now getting u1255 current, c1241 current, p404 history, b1552 history & b2120 history. engine check light still on, anything else i need to do?

[Cadillac Jim]

CURRENT

U1255 Class 2 Communication Malfunction (Serial Data Line Malfunction)

C1241 Variable Effort Steering Circuit Malfunction

HISTORY

P0404 Exhaust Gas Recirculation (EGR) Open Position Performance

B1552 Keep Alive Memory (KAM) Error

B2120 Lumbar Up/Down Sensor Failed

When you run the codes, the IPM polls the modules in order: PCM, IPM, ACM, SDM, TCS, RSS, PZM, IRC, RFA, CCP, MMM, MSM, PHN. Nearly all the codes are thrown by just one module but the network codes are different. The U1255 can be thrown by almost any module. Please tell us which module is throwing the code.

You can ignore the B1552 because this code is thrown when the battery is disconnected and goes away on its own. The B2120 is thrown when you use the lumbar support position control on the power seats to push the lumbar support to the limit of its travel and hold the switch. It should go away in time if it's HISTORY. If its CURRENT, pull the lumbar support away from the limit of its travel just a little bit and the code will go to HISTORY.

The C1241 may be a driveability or even a safety issue, but won't turn on the MIL. The P0404 is what is turning on the MIL and needs to be cleared for emissions inspection.

I haven't had this problem but I'm going by the 1997 FMS pages 6-452 through 6-455. The P0404 is thrown when the EGR valve gets sticky. Usually this is not something that happens every time or all the time, which is why the MIL light stays on when the code is HISTORY. If you want to diagnose the problem completely before buying anything, you need a scan tool that reads "freeze frame" data that is stored by the PCM when a code is thrown. The dealer has those and you can buy PC-based or high-end professional mechanic's scan tools that can read freeze frame data. Without a scan tool, and with a HISTORY code, you are pretty much left with assuming that the EGR valve is sticking. You can remove the EGR valve, clean and inspect it, and put it back. It's a decent DIY job, but be sure that you have the proper gaskets and materials on hand before you start.

The C1241 isn't treated in the 1997 FSM that I can find; if anyone has a reference on it, please step in and let me know where to look in the FSM for it. Section 8A-45 does have a schematic. C1241 is the MagnaSteer system. It's implemented by the EBTCM (the ABS/TC module, bolted to the brake manifold and pump assembly low in the left front between the engine and the radiator) and the steering rack-and-pinion. There is a WHT and a GRY wire. The EBTCM provides power to a solenoid in the steering column that changes the amount of power steering assist; you get more assist at low speeds for easy maneuvering and parking, and less at highway speeds for better steering feel and response. Check for breaks or shorts in the WHT and GRY wires, and in the wiring harness in general.

Chances are that connectors or wires were disturbed during the work to clear P1634, if these codes weren't present before that work was done. This can help locate the problems.

Be sure and run the codes again and tell us which module threw the U1255. If it's the IPM, it could be any module, but since P1634 was also thrown, my guess is that it's the EBTCM. Check the PPL wire in the wiring harness near the EBTCM while you are tracing the WHT and GRY wires and you will likely see the problem.