I apoligize for reposting, but I really would like to get this lady's car fixed! My name is Chip and I am a disabled ex-class A Tech. I have started a ministry out of my church where I work on (and hopefully fix!) peoples cars for no labor charges. Through donations to the ministry we even purchase the needed parts if the owner is really destitute. Eventually I hope to take in donated cars that need repairs and, after making them roadworthy, re-donate them to people who otherwise can't get a job or be self sufficient.(We have no mass transit here) I'm finding it to be a really rewarding experience to see the looks on their faces when I'm through and there's no bill. Now I have a question for you, if you would be so kind. I'm working on a '96 Grand Am with a 2.4 and about 126k that when I first got it was blowing BLACK soot and smoke out the tailpipe. It had a code of P0140, no activity on the downstream O2. After replacing a bad fuel pressure regulator and the plugs, I replaced the sensor, since it was so covered in carbon it never would have worked. The car now ran fine, mileage doubled, and we were all happy. I did tell them that the converter may have been damaged by running it for a while with all that excess gasoline going through, and they said they had already replaced it. I somehow didn't ask if it was replaced before or during the black smoke. Now comes the fun part: I got a call back from them that the CEL was on again, although the car was still running fine. I pulled DTC P0140 again, removed the sensor thinking that some remaining carbon had fouled it again. It appeared to be in good shape but since it was under warranty I replaced it again, then watched it on my scan tool. The voltage was at .440VDC which is in the right range, but it never changed even a thousandth of a volt, under any condition of load or throttle. I unplugged it from the harness with the engine running and the reading STILL said .440VDC! It didn't set a code or even flash the CEL or affect the running at all. I don't see how the scan tool could still be seeing a voltage reading when the source of voltage was removed. I checked the continuity of the signal wires to the PCM and for any shorts to power but could find nothing wrong. My sense now is that it must be the PCM, but I don't know how to prove it. Also the parts place says they keep the original PROM, they just put it into the new PCM. Please help me out, and tell me what to do. Thank-you for taking the time to read this, it's good to know that there are other people helping strangers. Chip from the TUMC Car Repair Ministry. eMail: snipped-for-privacy@yahoo.com P.S. There's a nagging thought in my head that says the Cat went bad again, but that wouldn't cause me to have a voltage reading with the O2 disconnected, would it?
Circuit Description In order to control emissions, a catalytic converter converts any harmful exhaust emissions into harmless water vapor and carbon dioxide.
The Powertrain Control Module (PCM) has the capability to monitor this process by using a rear heated oxygen sensor (HO2S 2). The HO2S 2 , located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust emissions effectively. If the catalyst is functioning properly, the HO2S 2 signal will be far less active than the signal produced by the front oxygen sensor (O2S
1).
Replace the entire HO2S 2 assembly. Do not attempt to repair the HO2S 2 if the following conditions exist:
Damaged wiring Damaged electrical connectors Damaged terminals Obstruction of the air reference and degraded HO2S 2 performance could result from any attempt to repair the above conditions.
The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.
Conditions for Setting the DTC DTC's P0106, P0107, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0301, P0302, P0303, P0304, P0506, P0507, P1406, P1441 not set. Engine has been running longer than 30 seconds. Engine Coolant Temperature (ECT) is greater than 40° C (104° F). Calculated Air Flow is greater than 14 g/s for 140 seconds. Throttle Position (TP) is between 5 % and 56 %. HO2S 2 remains between 425 mV and 460 mV for 125 seconds.
Action Taken When the DTC Sets The malfunction indicator lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the malfunction present. The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers. A history DTC stores. The coolant fan turns ON.
Conditions for Clearing the MIL/DTC The MIL will turn OFF after three consecutive ignition cycles in which the diagnostic runs without a fault. A history DTC will clear after 40 consecutive warm-up cycles without a fault. The MIL/DTCs can be cleared by using the scan tool.
Diagnostic Aids An intermittent could be the result of the following conditions:
A poor electrical connection Moisture in the electrical connector/wiring Rubbed-through wire insulation A broken wire inside the insulation Check for the following conditions:
A poor electrical connection or a damaged harness--Inspect the PCM harness for an open in the HO2S 2 signal circuit. Check for the following conditions: Improper mating Broken locks Improperly-formed connectors Damaged terminals A poor terminal-to-wire connection A damaged harness
Intermittent test--Observe the HO2S 2 on the scan tool while moving the related electrical connectors and the wiring harness with the ignition switch ON. If the malfunction is induced, the HO2S 2 display will change. This test should help isolate the location of the malfunction.
Replace the entire HO2S 2 assembly. Do not attempt to repair the HO2S 2 if the following conditions exist:
Damaged wiring Damaged electrical connector Damaged terminals Obstruction of the air reference and degraded HO2S 2 performance could result in any attempt to repair the above conditions.
The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.
Test Description Number(s) below refer to the step number(s) on the Diagnostic Table.
The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.
This step determines if DTC P0140 is the result of a hard failure or an intermittent condition.
Disconnecting the HO2S 2 and jumpering CKT(s) 1668 and 1669 to ground will determine if the PCM or wiring or HO2S 2 is at fault.
Determines which circuit the fault is in. If CKT's 1668 and 1669 are OK, then the PCM connection or PCM is faulty.
Replacement PCM's must be reprogrammed. Refer to the latest Techline information for programming procedures.
1 Was the Powertrain On-Board Diagnostic (OBD) System Check performed? -- Go to Step 2 Go to Powertrain On Board Diagnostic (OBD) System Check
2 With engine above normal specified operating temperature. Install a scan tool. Run the engine above the specified RPM for two minutes. Does the scan tool display a HO2S 2 voltage between the specified value? 80°C (176°F) 1200 RPM
425 mV-460 mV (0.425V-0.460V) Go to Step 4 Go to Step 3
3 Turn the ignition ON, with the engine OFF. Review the Freeze Frame data and note the parameters. Operate the vehicle within the Freeze Frame conditions and Conditions For Setting The DTC as noted. Does the scan tool display the HO2S 2 voltage steady around the specified value?
425 mV-460 mV (0.425V-0.460V) Go to Step 4 Go to Step 11
4 Turn the ignition ON, with the engine OFF. Disconnect the HO2S 2 electrical connector. Jumper the HO2S 2 high and low circuits (PCM side) to ground. Does the scan tool display the HO2S 2 voltage below the specified value?
100 mV (0.10V) Go to Step 5 Go to Step 6
5 Turn the ignition OFF. Check for a faulty connection at the HO2S connector (PCM side) and repair as necessary. Refer to Section 8A-5 .
Was a repair necessary? -- Go to Step 11 Go to Step 7
6 Turn the ignition OFF. Remove the jumpers and reconnect the HO2S 2 electrical connector. Disconnect the PCM connector. Probe the PCM harness HO2S 2 low terminal with a test light to B+.
Does the test light illuminate? -- Go to Step 8 Go to Step 9
7 Replace the HO2S 2 sensor.
Is the action complete? -- Go to Step 11 --
8 Check for an open in the HO2S 2 high circuit or poor connection and repair as necessary.
Was a repair necessary? -- Go to Step 11 Go to Step 10
9 Repair the open in the HO2S 2 low circuit.
Is the action complete? -- Go to Step 11 --
10 Replace the PCM.
Is the action complete? -- Go to Step 11 --
11 Using the scan tool, clear the DTC's. Start the engine and idle at normal operating temperature. Operate the vehicle within the conditions foe setting this DTC as specified in the supporting text.
Does the scan tool indicate that this diagnostic has ran and passed? -- Go to Step 12 Go to Step 2
12 Check if any additional DTC's are set.
Are any additional DTC's displayed that have not been diagnosed? -- Go to applicable DTC table System OK
The .44 volts is the bias voltage that the PCM applies to the O2 sensor signal line, normally this would indicate an open circuit in the O2 circuit. Recheck the circuit for continuity and don't forget to check for proper tension on the connector terminals.
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