Would a bad cat throw a code?
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our old cars do not have and after cat sensor like the E38 or E39, only pre-cat. So that sensor just reports back to the brain what comes out of the engine, but not what comes out of the cat.
i would check the MAF, that is in many cases the actual cause of the O2 sensor warning, because the sensor can only 'report' back what it gets out of the engine.
MAF sensor tap test
with the engine at idle speed, gently tap the MAF sensor with the fingers of an open hand, if the engine stumbles or stalls, the MAF sensor is defective.
http://www.mccd.edu/faculty/gregorya...0Sensors.ppt#1
http://www.fourwheelstampeders.ca/wi...php/MAF_Sensor
http://members.lycos.co.uk/piperboxster/maf.htm
http://www.carlton24v.co.uk/afmmaf.htm
http://www.bba-reman.com/content.asp...da_vw_vauxhall
An engine with a bad MAF sensor may be hard to start or stall after starting. It may hesitate under load, surge, idle rough or run excessively rich or lean. The engine may also hiccup when the throttle suddenly changes position.
If you suspect a MAF sensor problem, scan for any fault codes. A MAF problem should (but does not always) set a fault code.
Of course, don’t overlook the basics, such as low engine compression, low vacuum, low fuel pressure, leaky or dirty injectors, ignition misfire, excessive backpressure (plugged converter), etc., since problems in any of these areas can produce similar driveability symptoms.
MAF sensors can be tested either on or off the vehicle in a variety of ways. You can use a MAF Sensor Tester and tachometer to check the sensor’s response. If testing on the vehicle, unplug the wiring harness connector from the sensor and connect the tester and tachometer. Start the engine and watch the readings. They should change as the throttle is opened and closed. No change would indicate a bad sensor. The same hookup can be used to test the MAF sensor off the vehicle. When you blow through the sensor, the readings should change if the sensor is detecting the change in air flow.Another check is to read the sensor’s voltage or frequency output on the vehicle. With Bosch hot-wire MAF sensors, the output voltage can be read directly with a digital voltmeter by backprobing the brown-andwhite output wire to terminal B6 on the PCM. The voltage reading should be around 2.5 volts. If out of range, or if the sensor’s voltage output fails to increase when the throttle is opened with the engine running, the sensor may be defective. Check the orange and black feed wire for 12 volts, and the black wire for a good ground. Power to the MAF sensor is provided through a pair of relays (one for power, one for the burn-off cleaning cycle), so check the relays too, if the MAF sensor appears to be dead or sluggish. If the sensor works but is slow to respond to changes in air flow, the problem may be a contaminated sensing element caused by a failure in the self-cleaning circuit or relay. With GM Delco MAF sensors, attach a digital voltmeter to the appropriate MAF sensor output terminal. With the engine idling, the sensor should output a steady 2.5 volts. Tap lightly on the sensor and note the meter reading. A good sensor should show no change. If the meter reading jumps and/or the engine momentarily misfires, the sensor is bad and needs to be replaced. You can also check for heat-related problems by heating the sensor with a hair dryer and repeating the test. This same test can also be done using a meter that reads frequency. The older AC Delco MAF sensors (like a 2.8L V6) should show a steady reading of 32 Hz at idle to about 75 Hz at 3,500 rpm. The later model units (like those on a 3800 V6 with the Hitachi MAF sensor) should read about 2.9 kHz at idle and 5.0 kHz at 3,500 rpm. If tapping on the MAF sensor produces a sudden change in the frequency signal, it’s time for a new sensor.
If the readings jump all over the place, the computer won’t be able to deliver the right air/fuel mixture and driveability and emissions will suffer. So you should also check the sensor’s output at various speeds to see that its output changes appropriately. Another way to observe the sensor’s output is to look at its waveform on an oscilloscope. The waveform should be square and show a gradual increase in frequency as engine speed and load increase. Any skips
or sudden jumps or excessive noise in the pattern would tell you the sensor needs to be replaced. Yet another way to check the MAF sensor is to see what effect it has on injector timing. Using an oscilloscope or multimeter that reads milliseconds, connect the test probe to any injector ground terminal (one injector terminal is the supply voltage and the other is the ground circuit to the computer that controls injector timing). Then look at the duration of the injector pulses at idle (or while cranking the engine if the engine won’t start). Injector timing varies depending on the application, but if the mass air flow sensor is not producing a signal, injector timing will be about four times longer than normal (possibly making the fuel mixture too rich to start). You can also use millisecond readings to confirm fuel enrichment when the throttle is opened during acceleration, fuel leaning during light load cruising and injector shut-down during deceleration.
