Magazine for the auto service professional
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20 | February 2016 On many GM models it can be done with the sensor hot, and the PCM will relearn the real value at the next cold-start. Heater monitors When chasing sensor heater codes, frst make sure the heater monitors have run. On most models it's the frst monitor to run, and the other monitors might not run if that one doesn't or if it fails. Typically that monitor will run during a 40 mph to 50 mph cruise with the engine at normal operating temperature, but the monitor enabling criteria are different for each manufacturer, so it's important to look that up in the service information. The PCM monitors the oxygen sensor heater circuits for both voltage and current. On most models, the voltage monitor runs continuously and the current monitor runs at least once per drive cycle after specifc enabling criteria are met. Newer Chrysler models are a little different; the heater monitor runs after a complete (defned) drive cycle with the engine off and coolant temperature has decreased more than 60 degrees F (16 degrees C). The voltage monitor simply checks for battery voltage in the heater circuit. The current monitor checks to see if current in the circuit is within a specifc range. Depending on the model, it can range any- where from 0.2 amps up to about 8 amps or more. Most manufacturers list that spec in their service information, so you can check heater current yourself with a cur- rent probe. On AFR sensors, you can also see heater current duty cycle with an amp probe and a DVOM or with a scope, but you probably won't fnd a specifcation. When certain faults are detected, such as high current draw in the heater circuit, the PCM will suspend heater operation to protect itself. This can be a challenge to diagnose correctly, because if the PCM does not ground the heater circuit, it can be hard to tell if the problem is in the circuit or the PCM. Here's a test you won't fnd in any service manual. Unplug the sensor and connect a small light bulb to the heater circuit. A side-marker bulb will do nicely, and make sure you're connected to the heater circuit and not the sensor circuit. Clear the codes and turn off the MIL, then turn on the igni- tion switch. If the bulb lights or pulses, the circuit has power and the PCM is attempting to operate the heater. The generic OBD-II codes for oxygen sensor heater malfunctions are useful, and the manufacturer-specifc codes are even more helpful. Still, the real challenge is telling the difference between a failed sensor and a faulty PCM. The key to that is understanding the control strategy, diagnostic monitors and enabling criteria, so start by looking up the description and operation and searching for TSBs in your service information system. Then proceed with confdence. ● Powerplant Courtesy of Snap-on Solus Ultra With the scan tool reading enhanced (OEM) data on a 2013 Acura, the tool reports heater duty cycle and sensor plus/minus signal voltage. Control current for the oxygen pump cell can fow in either direction depending on what's required to keep AF lambda at 1.00.