I get quite a few help request emails from people through my website. A surprising number of them will rant about how a customer went to a shop, was charged $800, went to a second shop when the vehicle wasn’t fixed, was charged another $500. They went to a third shop because the second shop also failed to fix the concern, and finally wound up at a shop where they were given a $600 estimate but were afraid to give the green light. They want my opinion as to whether the third shop’s repairs would take care of the concern.
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What do you tell somebody like that? In this consumer-driven world, folks tend to take their business elsewhere if they don’t like the merchandise or service, and rather than giving the first shop a chance to straighten things out, they just try another shop.
I usually ask, “Why didn’t you take it back to the first shop and have them make good on their repair instead of bouncing to another shop and then another?”
They almost never reply to that question. Granted, some of them tell me they have gone back to the first shop only to be told that more money needs to be spent to fix the same problem, and that kind of thing is frustrating beyond words. Who wouldn’t lose confidence in a shop that pulls that kind of stunt?
From the shop owner’s/technician’s perspective, sometimes it seems like the deck is stacked against us. At the Ford dealer, I once drew a ticket on a mid-1980s Ford pickup that barely had the hood opened since it was new. It was a rough-idle-stall issue, and I found clogged idle air control passages and throttle body, filthy gasoline-laced engine oil, spark plugs that had almost melted completely away at their tips, plugged fuel filter, late ignition timing, a noisy TP sensor signal and a bevy of vacuum leaks. We created an estimate, received the go-ahead, made the repairs and test-drove the truck (which ran very well), but a week later he came back and threw the “my truck still ain’t fixed” flag. I discovered on that visit that the lift pump in the fuel tank usually was dead and that when the fuel tank was low on gas, the pressure pump on the frame might or might not begin to lose its prime. From 1985 until 1989 Ford Pickups had that two fuel pump system.)
I hadn’t sold the guy anything he didn’t need on the first trip, but when I gave him the estimate for replacing the dead in-tank fuel pump, he wanted me to guarantee that the fuel pump was going to fix his ragged, rusty, bald-tired ill-maintained ride. What would you tell him?
Paying Tribute
Larry’s son lives a few hours to the north of us, and he drives a Mazda Tribute, which had no symptoms other than an illuminated Check Engine light. Because the son was in town, Larry brought the Mazda to have us look at it. My guy promptly pulled a P0135 code, which points to an open in the HO2S 1/1 heater circuit, and while pinpoint tests abound for this kind of concern, it isn’t rocket science to troubleshoot an O2 heater.
First, though, it’s wise to make sure you’re checking the right sensor, particularly on a vehicle that has two upstream and two downstream sensors. Back in 1994, I botched that up on a brand new Thunderbird when I kept checking the wrong sensor because I didn’t know which sensor was which. In those early days of OBDII, I was erroneously thinking 1/1 and 1/2 should be the front sensors and 2/1 and 2/2 should be the rear ones. Everybody knows how wrong that is now.
HO2S 1/1 is upstream of the catalyst on the same bank of the engine where cylinder No. 1 is located, and HO2S 1/2 is the downstream sniffer on that side. That bank isn’t on the same side on every V-engine vehicle. On this Mazda, the rear bank is where the number one spark plug is — the No. 1 cylinder is the one attached to the front connecting rod on the crankshaft on every engine I’ve ever seen, and on GM and Dodge engines, the No. 1 cylinder is on the opposite bank from Ford-Mazda packages.
With a heater code like this, the sensor itself is usually the problem. But as we investigated, we found that somebody had already screwed a new one in there. The O2 sensor connector was pretty and white, which didn’t match the green harness connector in color, but that in and of itself wasn’t a problem. What I did wonder about was who screwed the new sensor in, how much they charged and why they gave up after replacing the part. His son isn’t that wrench savvy — this sensor is tough to get to. Maybe he took it to a parts store, got a sensor and had a wrench-friend screw it in? Larry hadn’t a clue, and I didn’t press the issue, because there was no point in it.
Anyway, we started by measuring the resistance of the O2 sensor heater. Those two wires will just about always be the same color on the sensor pigtail. The heater is usually fed by two white wires, two brown wires or two black wires. On this one we found 5.5 ohms, which is fairly normal.
Vectoring In on the Concern
The next thing we did was to utilize one of my own inventions, a handy dandy homemade tool. I take a matching O2 sensor connector (cut from an old sensor) with a light bulb wired into the two sensor wires. This is a cheap tool to build and always gives reliable results. With the engine started, the sensor heater should be powered and grounded and the light should illuminate. With that plugged into the green harness connector, we started the engine to see if that light would come on. If it did, we’d know we had both power and ground at the sensor. It didn’t illuminate, and that took us one step closer to the repair, but we hadn’t arrived at our final prognosis.
We found that the sensor had good 12-volt power available to the heater so we didn’t need to go that way, but the PCM delivers the ground (and measures O2 heater current draw), so we switched our focus to the dark side. I explained to my student that while most learners are quick to check the bright side of a circuit, they have to learn (using my Darth Vader voice) that you don’t know the power of the dark side. Grounds frequently are overlooked, but this ground is a controlled output from the PCM, and here we were.
From the PCM to that O2 sensor is a straight shot – no connectors en route, and so we drew on our ALLDATA wiring schematic to find that pin 93 feeds ground to the O2 sensor heater on this one.
With the PCM disconnected, it was a simple matter to first examine pin 93 for damage or push-back, but it looked brand new. Then we measured current carrying ability from cavity 93 to the wire right there at the connector, checking for a bad crimp or maybe some oxidation, but that crimp was solid. The PCM is only about 24 inches from the O2 sensor connector and the circuit is cocooned in convolute loom. There was no continuity between pin 93 and the appropriate O2 sensor connector pin.
This vehicle’s wire harness looked like factory – nobody had hacked around behind the engine and there were no chafing or burning concerns, so how this perfectly undisturbed harness could have developed an open circuit in just one of the myriad of wires traveling through it represents something of a mystery. The detective in me wanted to split the harness and find out what the heck was going on in there, but the production-oriented mechanic in me decided to have Daniel run an overlay through a small piece of split loom secured to the harness. A properly run overlay is the smart way to handle this kind of thing. Solder, heat shrink, loom, wire ties (preferably black ones), etc. and we had a closed circuit heater once again. Check Engine light out – current readings on the scan tool normal. Mission accomplished. This was a textbook troubleshooting and repair operation.
Other Jobs Under Way
As is usually the case, there were other jobs under way in my shop – a 1996 Taurus that was purchased as a used car that immediately developed overheating and transmission concerns (cooling system loaded with rust and intermediate clutch pack fried), an Expedition that had a leaking heater core (replaced two years ago for leaking but was leaking again).
The Expedition was fun, and while the guy I gave it to is fresh out of high school, he has at this point completed most of my courses as a dual enrollment student and he’s cruising toward a profitable career as an automotive technician. I have him working afternoons at a very busy independent shop.
That’s a story in itself – the owner of the shop called me to see if I had anybody he could use. He aired his frustration at the fact that every mechanic he has hired for the past two years has disappointed him. They start out strong but after a month or two, just about every one of them turns lazy, unprofitable and undependable. He explained that he had to turn work away, because he didn’t have sufficient help to get it out of the shop. I sent him Derrick last fall, and he couldn’t be happier with this young fellow’s work energy, enthusiasm, work ethic and willingness to take on virtually any job.
Derrick puts in his Monday through Thursday mornings with me, and his afternoons, Fridays and some Saturday mornings at the shop where he works.
He dug into the Expedition heater core job with knowledge and understanding because he had done the job on other Expeditions at the garage where he works. When Derrick got the instrument panel and air box cover off to expose the heater core, he pointed out that the blend door had been improperly installed by the previous technician (no wonder the heat didn’t work) and the rusty water that was leaking out of the core indicated that there was probably some electrolysis going on. A vigorous cooling system flush would be in order. But there was more.
The replacement heater core was different from the heater core we were removing. It was outfitted with a restriction in the heater core inlet, and when I saw that, I cautioned Derrick to make sure he put the feed hose on the heater core tube with the restrictor crimped into it. The return flow shouldn’t be restricted or the heater core will burst from internal pressure. Interestingly, there were no instructions with this replacement heater core regarding the restrictor. Because both tubes are the same size, it would be easy to make a comeback-generating mistake. The restrictor typically is there to reduce noise, and it also allows the water to travel more slowly through the core so as to release more heat.
The feed hose generally comes from the water pump, but we wanted to be absolutely sure which way things were flowing on this one, so we used a piece of clear hose from Ace Hardware and looped out the two hoses. When we started the engine we saw which way the coolant went – there was no guessing and we also were able to determine that we had good coolant flow – a rusted out water pump impeller might keep the engine cool but move very little through the heater core.
The core was replaced, the quick connect fittings were replaced, and I personally took the Expedition to the wash rack and ran clean water from a hose through both heater cores, flushing out enough rust to paint a parking lot. With everything reassembled, the rest of the cooling system flushed with the big machine and a 50/50 coolant mix flowing, we saw a 175-degree temp ceiling. Replacing the thermostat raised it to 200 degrees, which was much better. That and a blend door finally got some heat flowing.
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