P042 Code - Left Oxygen Sensor Signal Lean

[Ed Hall]

My gas mileage is not as good as it should be on this car. It is using about 20% more fuel than my 1988 Eldorado. This morning while driving very easy at around 40 mph, the check engine light came on so I checked the DIC and it gave a (current) PO42 code - Left Oxygen Sensor Signal Lean. So does this mean that the oxygen sensor is sensing lean conditions and is making things too rich resulting in poor fuel economy? Should I buy a new oxygen sensor? The car just turned 64k miles.

[cadillac360]

It may just need to be cleaned. I have a 96 deville with 165k miles and never had a problem with my sensor. All you need is a bottle of electrical parts cleaner, which you spray on, its easy. Im no expert or anything, but I would try that. I think the spray is pretty cheap, less than ten bucks.

[Caddynut]

Just had the same exact code and problem with my '92 Seville. I had one of my Oxygen sensors go bad. I replaced them both and it runs better than it ever has.

[BodybyFisher]

Ed here is an interesting article that I saved. If you can get into the DIC you might be able to monitor the O2 sensor data:

http://www.aa1car.com/library/o2sensor.htm

[The Fred]

On my 1990 Seville you can monitor the O2 sensors through the dic

[BodybyFisher]

This is from Autotap:

SENSOR DIAGNOSIS

O2 sensors are amazingly rugged considering the operating environment they live in. But O2 sensors do wear out and eventually have to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor can also be damaged by environmental factors such as water, splash from road salt, oil and dirt.

As the sensor ages and becomes sluggish, the time it takes to react to changes in the air/fuel mixture slows down which causes emissions to go up. This happens because the flip-flopping of the fuel mixture is slowed down which reduces converter efficiency. The effect is more noticeable on engines with multiport fuel injection (MFI) than electronic carburetion or throttle body injection because the fuel ratio changes much more rapidly on MFI applications. If the sensor dies altogether, the result can be a fixed, rich fuel mixture. Default on most fuel injected applications is mid-range after three minutes. This causes a big jump in fuel consumption as well as emissions. And if the converter overheats because of the rich mixture, it may suffer damage. One EPA study found that 70% of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.

The only way to know if the O2 sensor is doing its job is to inspect it regularly. That’s why some vehicles (mostly imports) have a sensor maintenance reminder light. A good time to check the sensor is when the spark plugs are changed.

You can read the O2 sensor’s output with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers jump around so much. Here's where a PC based scantool such as AutoTap really shines. You can use the graphing features to watch the transitions of the O2 sensors voltage. The software will display the sensor’s voltage output as a wavy line that shows both it’s amplitude (minimum and maximum voltage) as well as its frequency (transition rate from rich to lean).

A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (within 100 milliseconds) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor’s output to drop to its minimum (0.1v) value. If the sensor doesn’t flip-flop back and forth quickly enough, it may indicate a need for replacement.

If the O2 sensor circuit opens, shorts or goes out of range, it may set a fault code and illuminate the Check Engine or Malfunction Indicator Lamp. If additional diagnosis reveals the sensor is defective, replacement is required. But many O2 sensors that are badly degraded continue to work well enough not to set a fault code—but not well enough to prevent an increase in emissions and fuel consumption. The absence of a fault code or warning lamp, therefore, does not mean the O2 sensor is functioning properly.