You’ve been here before. A vehicle’s ‘Service Engine Soon’ light appears and the customer is looking for answers. You get out the diag-shovel and start to chip away. First stop is a quick scan of the diagnostic trouble codes. A seemingly simple P0xxx performance code. The diagnostic chart says that this code can be caused from the engine computer noting an unexpected drop in sensor signal. You hop over to the data stream to see what all the fuss is about. Then, without warning, a flash of lightning and a booming thunder clap. Your eyes start to cross as a rotating blue gear appears on the screen and soon after....your worst nightmare is spelled out: Technician A says the scantool data stream can always be trusted. Technician B says to check a questionable signal at the sensor. Who’s right? Oh no! They said this would never happen in real life and those types of test questions aren’t realistic. This can’t be happening! Your thoughts race... What about technician ‘C what google says’? Or technician ‘G, isn’t it time for lunch’? You take a gulp of your energy drink to flush out the Prometric-Panic and begin to regain your focus. Now operating on stimulated cerebrospinal fluid, your brain is reminiscent of battles won in similar times. You remember that precision electrical diagnosis must rely only on facts from accurate testing.
As diagnostic flow charts go, the steps are often very similar. Step one, duplicate the conditions and monitor for the concern to return. Step two, see what the computer sees in the moment of failure. Step three, verify the computer is getting an accurate signal. Step four, verify the circuit to the component. Step five, replace component and retest. Sounds simple but, as you know, not all problems play by the rules.
A vehicle arrives with a P0161 - Oxygen sensor heater (Bank one, Sensor two) performance. The friendly neighborhood auto parts store has already sold the customer all four oxygen sensors, just to be safe *rolls eyes*. The customer, beaming with a sense of accomplishment, has replaced the sensors with the help of the internet, touched the battery cables together (clearing the codes) and driven the vehicle for two days since the busy weekend. Now the customer, a couple hundred dollars lighter with bruised ego and scraped knuckles, is asking you to check out why the ‘Check Engine’ light has returned. The customer’s wife is standing behind him with arms crossed and irritated that she’s going to be the family Uber while his car is down for the count. You start with a quick scan of the system to verify what the computer sees. You see it is the same code the customer was fighting several days previous. The possibilities begin to roll through your mind of the dozens of things that could be wring. All you know for a fact is that the sensors are new but are of questionable quality. Seems unlikely that a ‘new’ sensor would have the exact same problem, right? RiGhT?!? You click through the service manual and find the diagnostic flow chart. Step one, access the oxygen sensor heater (bank two, sensor two) and measure the resistance of the heating element. Side note: The service manual’s first step is to go right to the sensor for testing. It doesn’t ask to check the data stream or even check the resistance at the computer connector. It wants you to check for information/facts at the scene of the crime. Now back to it. You click your digital multimeter over to the horseshoe (ohms/resistance) and carefully probe the two terminals at the sensor that lead to the heating element. The DMM blinks some numbers and ranges bounce around but settles on 4.1ohms. Compared the specifications in the service manual, this is considered good. On to the next step which is designed to test the circuit, or wires, that lead to the sensor. To break it down, the manual wants you to check to see if the wires are continuous (or have low resistance), are shorted to ground, or are shorted to power. After a few more swigs of energy drink and referencing a spaghetti maze of schematics, you have isolated the wiring to and from the sensor. One of the two wires travels to a near-by ground point on the engine block. You test this wire for resistance. You find the wire to have very low resistance from the sensor to ground. Good, One down and one to go. The other wire, you soon learn, was used by engineering to build the entire vehicle around but you find it eventually leads to the engine computer. You disconnect the massive wiring connector that has at least 5,289 pins from the computer and carefully inspect the connector and its terminal numbering system to determine which wire leads to our troubled sensor. Pin 10, according to the wiring diagram leads directly to the oxygen sensor heater in question. You horseshoe-test the wire from end to end and find the wire has very low resistance as well. Interesting. The heating element is good. The wiring is good. There’s only one other thing it could be. The dreaded “PCM HAS FAILED, MUST REPLACE” diagnosis. Oof. Sounds expensive. And the more expensive, the higher the liability. How sure are you? ‘What if’ scenarios begin to roll around your head like a ball on the roulette wheel. The service manual says that PCM replacement is required. Who’s to disagree with official printed information? You dig deep into the memories of electrical diagnosis class that bored you to sleep and you come up with an idea. It’s a crazy idea so hopefully nobody sees, but if it works, it will verify the PCM failure. You dig through your junk drawer and find a headlight bulb. Headlight bulbs take a good amount of power to illuminate. You roll underneath the vehicle back to the oxygen sensor heater wiring. You carefully attach the two heater element terminals to the headlight bulb. Over at the disconnected PCM, you locate pin 10 again. Here goes, moment of truth. You apply a fused power supply to the harness at pin 10. According to the wiring diagram, this wire should go to the heater element, through the element, and another wire completes the circuit to ground. If your energy drink focused calculations are correct, the light bulb should light up, in place of the element, when power is applied to pin 10.
...Nothing. The light is as dark as this job is turning out to be. Why? The wires checked out with the DMM. Weird? With power still applied to pin 10, you start to poke around in the area where the engine harness is routed. After a few quiet seconds, the light bulb begins to flicker and continues to flicker the more you wiggle the harness around. After omitting demon possession from the possibilities, you notice that just out of sight, the wiring harness has some damage. Looks like a rodent of some kind has nibbled his way into the ONE WIRE that you’re chasing. If the wire is twisted, the circuit is able to make enough contact and carry to the load to the head light bulb. Success! While others were laughing and the service manual was very clear on the next steps, you went the extra mile and load tested the wiring to the heater. The damaged wire had enough conductor remaining to pass a resistance test from a DMM but not enough to pass actual power to the heater element. Your request for a wiring repair is approved and you’ve saved the day again. That was a close one. You almost condemned a $800 computer and risked the gut-punch feeling of a legitimate comeback.
Accurate testing requires the technician to go the extra mile. Digging a little deeper and sometimes thinking outside of the box is what separates the DIY crowd from the trained professional. Testing each component of an electrical circuit thoroughly and avoiding tunnel-vision diagnosis is required. Gaining facts and even testing the reasons you consider them facts is essential. In a world of ‘parts-cannons’ and wild guesses, be the one who knows for sure.