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Recently my check engine light has been cycling on and off on my 1999 Deville D'elegance with (P0101) mass or volume air flow, (P0151) oxygen sensor circuit/low voltage bank 2 sensor 1, (P0404) Exhaust gas recirculation control circuit range, and (P0741) torque converter clutch solenoid.

The car seems to run fine most of the time although it looses power or won't maintain speed on big hills but I recently found out part of my throttle body was disconnected and that seemed to fix that problem. Other than that it won't shift correctly but very rarely when the check engine light is on. The previous owner told me the computer was bad on the car so I don't know if the parts are actually bad or is it just the ECU. I think it might be the ECU because evertime the check engine light comes on it throws different codes. Only code that had been consistent is the torque converter one. Any advice would be helpful before I start throwing money into it

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When the PCM sees a problem with the drivetrain, it takes actions to ensure that the car is still functional until the driver/owner can get it serviced. That includes disabling adaptive shifting, closed-loop DFI, and sometimes other things like disabling the cruise control, A/C, etc. as needed to protect the car from further damage. If the check engine light has been coming on, these problems affect the emissions of the car and can be a problem in getting it inspected, or there is some possibility that the problem will cause damage if left untreated.

The codes you cite are

P0101 Mass Air Flow (MAF) Sensor Performance

P0151 Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 2 Sensor 1

P0405 Exhaust Gas Recirculation (EGR) Position Sensor Circuit Low Voltage

P0741 TCC System Stuck Off

The first three will turn on the check engine light when the codes are CURRENT but not when they are HISTORY. The best time to check the codes is after you have driven the car and seen the problem, but before you turn the key off, so you can see which codes are CURRENT. Once you turn the key off, all the codes go to HISTORY until the problem is detected again.

You should get the problems from the first three codes fixed before you bother with the transmission code. That particular code may mean that the TCC solenoid is bad, or that the transmission needs new seals and a torque converter, but driving it that way won't hurt anything other than loss of a MPG or two. Without the TCC the transmission will run hotter on the higway, so you might want to look at fixing the transmission before you do any long-distance towing.

My car is a 1997 Eldorado so my factory shop manual (FSM) doesn't apply to your car, although it does cover OBD II and the Deville for 1997, so it does apply to your problem. I'll make note of anything that might not be accurate for your 1999 Deville D'Elegance.

First, if the PCM (what the previous owner called the ECU) was bad, the car would not run. In fact it would not crank over. The whole car is controlled by computers on a network and very little or nothing runs on just switches and the battery.

You should never just throw parts at any car. Your can has an OBD II system that supports us in helping you fix just about anything. There are those here far more qualified than I am that can help you when things get really definite. I can help you get to the root of the problem.

From the codes you posted, you have at least two or three DFI problems and possibly a transmission issue. Let's start with what may be the root of the problem, the P0101 code.

According to my FSM, page 6-301, the PCM throws P0101 when the output of the mass airflow sensor (MAF) is too far from what it should be according to the intake air temperature (IAT), engine RPM, and the manifold air pressure (MAF) sensor. When the PCM throws this code, it drops the MAF signal and uses the computed value instead, the EVAP system is disabled, and the PCM management of the transmission shifting is disabled other than the adaptive shifting, which no longe updates, and the TCC is disabled. This is consistent with "part of my throttle body was disconnected." You should make sure that the throttle body is intact, that all the vacuum hoses are connected and in good shape, and the rubber body between the throttle body and the manifold is not cracked. Once the P0101 code goes away, most of your problems should be solved.

My FSM, page 6-372, says that a very lean condition such as that caused by leaks in the throttle body can throw the P0151 code. I would ignore this code until the throttle body was good to go.

My FSM, page 6-456, says that P0405 is thrown when the EGR valve position, as sensed, is outside the range normally seen for this sensor. This is the GRY wire on the EGR connector, and if it is outside the range 0.32 Volts to 3.69 Volts then the code is thrown. This may mean that the connector is loose, or that the EGR valve is stuck. First, check the ECS Fuse, 10 Amps (red) in the Engine Compartment Fuse/Relay Center, which is under a plastic cover in front of the strut tower on the driver's side. If the fuse is cracked or bad then replacing it may fix this code. Otherwise you need to look at the wiring harness connector to the EGR valve or the EGR valve itself.

-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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