Last month, we kicked this series off by showing you how to save some money by diagnosing a bad MAP sensor rather than just chucking cash at new parts that may or may not fix a specific problem. This month, we’re moving downstream to diagnose a bad O2 sensor.
Oxygen Sensor Malfunction
The oxygen sensor is a heat-activated sensor downstream in the exhaust system that monitors the air/fuel ratio via a chemical reaction with the exhaust gasses. In dumbed-down parlance, the O2 sensor “sniffs” the exhaust, and depending on the air/fuel ratio levels, converts that reading into a voltage signal that’s fed to the ECU. The ECU reads this voltage and either adds or removes fuel from the intake charge to try to maintain an ideal 14.7:1 air/fuel ratio. Readings below 0.3-volt tells the computer the mixture is rich, while readings above 0.6-volt tells the computer the mixture is lean. The O2 sensor won’t pass voltage until the element is heated by the exhaust gasses, so some O2 sensors will have a built-in electrical heating element to bring them up to the 600-degree Fahrenheit required to begin functioning. They’re generally long-lasting buggers, and when they go bad, it’s often due to other variables, such as sticking or leaking injectors, an intake or exhaust leak, or other mechanical faults upstream of the O2 sensor.
An oxygen sensor that’s going bad or has failed altogether won’t always throw a check engine light or trip a diagnostic trouble code. Sometimes if there’s an internal problem with the O2 sensor like a short to ground, the heating element malfunctioning, or some other electrical or mechanical issue you’ll be able to see in OBDII vehicles DTC P0030, P0031, or a range of codes between DTC numbers P0130-P0161. But most of the time the O2 sensors just get tired and stop accurately “sniffing” the air/fuel mixture, which can result in an air/fuel ratio that’s either overly lean (pinging and knocking) or more commonly, too rich (excessive fuel smell, poor mileage, hot or damaged catalytic converter).
You sometimes can test an O2 sensor in the vehicle with the engine hot (normal operating temperature) just after shutdown, but for our purposes (and more accurate testing) we’ll bench test the unit. After removing it with a dedicated O2 sensor socket available at most auto parts stores, clamp the sensor in a vise and set your multimeter to DC millivolts (mV) scale. Depending on your O2 type, you’ll see anywhere from one to four wires. The black wire is the signal wire, so insert one probe to the signal wire and the other probe should ground the body of the O2 sensor. With a propane torch, heat the O2 sensor element to just cherry red and then immediately back off with the heat, repeating this cycle off and on while watching the voltage reading. The sensor should cycle between 0.10-1.0 volts roughly three times per second if you’re using an analog meter. The digital meter will still show variances in the reading, but overall it won’t be as jumpy as the analog. With the torch applied, the oxygen is mostly removed and you’ll see the voltage hop up the scale. With the torch removed, you’ll see the voltage come down. Essentially, you’re looking for the variance between 0.10-1.0 volts to ensure your O2 sensor is working.
Like a MAP sensor the only fix for an O2 sensor that isn’t reading at all or is reading outside of its designed range is replacement. Once again for this series, we went to Quadratec to select from virtually any high-performance aftermarket or high-quality OE-spec replacement part for our Jeep. In our case, our O2 sensor had gone bad, reading out of range upwards on the rich side. We had been hearing a bad exhaust leak under the hood for some time, so to verify our suspicions we yanked the 250,000-mile factory exhaust manifold and cut away the heat shielding and found that the entire manifold, front and back, had deep fissures and spider cracks all over it. These cracks would be more than enough to introduce enough oxygen into the system to cause the O2 sensor to give a false reading to the ECU. The computer kept dumping more and more fuel to correct for what it perceived as a lean condition until our O2 sensor and catalytic converter were damaged.
Parts and Special Tools
Quadratec offers replacement parts for all Jeep models from current JK all the way back to the ’41 Willys in addition to certain specialty hand and diagnostic tools. Our tools were part of our current inventory, but if you don’t want to buy special diagnostic tools, your local auto parts store will sometimes rent or loan what you need.
• Multi-meter with DC scale
• Propane torch
• O2 socket
• Quadratec PN 55120.03 O2 MAP sensor for ’87-’90 Jeep with 2.5L engine