Jason writes:
Sajeev, I have a 2002 Mercedes C320. I replaced the pre-cat O2 sensors at 94k miles as recommended by my service technician. This was in the summer of last year, so roughly a year and a half ago. Since then I have had the CEL twice for O2 heater circuit malfunction on the pre-cat sensor on the driver’s side. The first time I replaced the sensor and that was six weeks ago. Yesterday I start the car up and it’s back (verified at the Advance store with OBDII). Have I received two bad O2 sensors in a row, or could I have something going on with the car that is causing the O2 sensors to have problems?
Sajeev answers:
First off, let me express my supreme disappointment with our society’s collective inability to discuss engine troubles in the appropriate OBD-II error code. It adds doubt, mistrust, and possible financial misconduct in whenever a warning light illuminates. Mechanics should tell customers they have a such-and-such code. Customers should hit the forums to learn more, namely the diagnostic tree to find the code’s problem. Forget about World Peace: everyone needs to talk in code.
So we need the code to answer the question, but perhaps it doesn’t matter this time ‘round. Codes relating to O2 sensors don’t always mean the O2 is bad. Sometimes an “O2 bank lean” code stems from a bad vacuum line in an emissions system. Or, even more obscure, a failing connection from the engine to the PCV valve. Yes really, that has happened.
Not that I fault your mechanic, it is wise to replace O2 sensors at this mileage. But there are other items to address, because replacing an O2 sensor is sometimes like shooting the messenger. I will not go into further detail, because websites like OBD-codes.com are a far superior resource. Once you get that precious code, that is.
(Send your queries to mehta@ttac.com)
ScangaugeII for the win!
On the mistrust angle, I think the damage is done. I’ll NEVER bring a CEL to a dealer or mechanic before getting a list of the codes before-hand.
If anything, it’s the one time you can check the mechanic’s work when he tells you what is wrong with it . . .
OBD-II codes are a good start to finding a problem, not the end. Go to the manual and follow the diagnostic tree before changing any parts. I was an ignition component product/application engineer on the GM 4.3L when OBD-II was introduced in 1995MY…..improper or lack of using a diagnostic trouble tree was a huge reason for misdiagnosed warranty. Resulted in many customers with multiple repair trips for the same code.
This reminds me of one of my many pet peeves with modern cars: Why doesn’t the car give more information to the user. “Check engine” doesn’t tell me if I need to fully tighten the gas cap, casually set up an appointment with my mechanic whenever it is convenient, or immediately pull over to the side of the road and turn off the engine. Would it be so hard to show on the dashboard LCD “Error code 123” so that I could then look that up in the user manual and find out what I should do. Now, about those user manuals that are written by lawyers…
Since memory is cheap, why stop at displaying just the code but also a plain English text summary of the problem is?
Its a vast conspiracy between automakers and independent service providers to keep the general public clueless. Thanks to the Internet and high fructose corn syrup, we have the attention spans of fruit flies, which cannot possibly understand the concept of an OBD-II warning code. And when you look at the lopsided profits a dealership gets from service departments relative to new car sales…
…well, we’re through the looking glass here people.
And where’s my tinfoil hat?
Malfunction lights do tell you when to shut down, but it is encoded in the lights color.
If the light is orange, the fault is “of concern but not engine damaging”. If the light is red, the problem will either damage the engine or shortly disable the vehicle- so shut things off as soon as it is safe to do so.
As always, there is one exception to prove the rule- If the orange light flashes, it means the engine is producing catalyst damaging misfire, and while the engine will keep going, expensive catalyst damage may occur. I’d treat a flashing orange light as if it were a red light.
I’ve told my wife to use the stop light rules- exercise caution with a yellow (orange) light, but if the light turns red, stop.
“Thanks to the Internet and high fructose corn syrup, we have the attention spans of fruit flies”
Mr. Mehta,
I am appalled at your lack of sensitivity towards the fruit fly community. As the proud parent of several thousand fruit flies, I can tell you in no uncertain terms that no child of mine has ever spent hours watching videos of piano-playing cats on YouTube or fine tuning an avatar on Second Life. Comparing the mental focus of Drosophila to that of humans is insulting to all fruit flies… Oooh, a banana!
Our (now gone) effing Kia said NOTHING in the user’s manual about checking the gas cap. We got a CEL, dropped it off at the dealer who told us they’d charge us for reading the code for a loose gas cap ????
Turned out to be an actual malfunction . . . (fixed under warranty over several days of foot dragging) duh. I was ready to fight since I’m the first person to grab the user’s guide and figure out what I’m supposed to do . . . my CR-V had a specific light and procedure for that particular issue and it was right in the user’s guide.
Sajeev, you’re beginning to sound like one of the B&B . . . it’s scaring me :o
If the light is orange, the fault is “of concern but not engine damaging”. If the light is red, the problem will either damage the engine or shortly disable the vehicle- so shut things off as soon as it is safe to do so.
What car is this? Intereting, all the cars I have owned up to and including my 09 Subaru just have the one orange check engine light.
I know alot of cars are getting a “check gas cap” light now since the loose gas cap sceneario seems to have been a source of frustration since the requirement for the evap emissions systems to be self diagnosing took effect.
The heater circuit can be tested directly, which clearly needs to be done in this case. A good scantool should tell you the circumstances of the failure, which can make it easier to reproduce.
Given the history of sensor replacement, chances are its the wiring or connector on the car side. Its also possible its the sensor on the opposite side of the engine. I’ve definitely seen scantools confuse them.
edit: Something else to consider is the brand of sensors used as replacements. If the resistance of the circuit differs slightly, the ecu may interpret it as in intermittent fault.
Amen to that. My dad had a 1982 Chevy Celebrity (later mine) that ate O2 sensors at the rate of about every 6 months after the first one failed. I often wonder if it was “cheap parts” (my dad was not a purchaser of Genuine Goodwrench parts) or a short circit somewhere. The orginal part lasted 100,000 miles.
BTW I know that’s the only good thing I’ve ever had to say about that car on here. My next vehicle purchase will be followed by buying the appropriate scan tool.
My thoughts EXACTLY. I envisioned factory trained technician BEAVIS yanking and yanking on the 02 sensor because it wouldn’t come out, then later twisted the ripped wires back together and covering the “repair” with electrical tape
Considering the cost of a Mercedes, we have two, I should not have to know about codes at all, much less what they mean. Our ML 320’s recent behavior has been bizarre at 70,000 miles, and the techs at MB don’t have a clue. So I will learn.
136K miles, 10 years later I ain’t touchin the O2 sensors on my e46 5 speed… never mind the buzz about replacing them every XX miles. Never had a CEL light, never had a single warning light other than low windshield wiper fluid once. I don’t care if it has 11hp less or runs a bit lean or a bit rich when there’s a full moon. Still pulls stronger than my brother in law 330 autobox and gets me 27.7 mpg combined.
Damn, newer knew that BMWs have “wiper fluid”, must be German engineering.
You can FIND the reservoir for the wiper fluid on an ML!?
You EVEN HAVE a LIGHT for wiper fluid being low on the dash?!
Hey B&B, I’m the Jason who asked the question. Update since I emailed Sajeev:
The code is P0155 Oxygen Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)
It also comes with another code for the thermostat:
P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
On the MB site I go to a lot (not sure if I’m supposed to name names here), the best we’ve come up with is that the car starts in the cold (Snowpocalypse here in VA) and the thermostat stays open allowing coolant to flow and the engine does not warm up fast enough, then the ECU tells the car to keep running rich to warm it up, then the O2 sensor code trips. I don’t get either code independent of the other.
Thanks in advance, and just to be clear, all of this has been DIY, so my mechanic didn’t rip me off (but maybe I did some unnecessary work) and the replacement sensors are Bosch just like OEM.
Thanks!
Thanks for writing to Piston Slap! Your DIY skills point to something I’ve mentioned before: readers of this series are far from the average motorist. Bravo.
With that info, I’d look at the coolant code first, and the OBD site I mentioned before has a good diagnostic tree. Testing sensors is the key before replacing the thermostat.
http://www.obd-codes.com/p0128
I’d wager that you need to test nothing if your heater is pathetic in this climate. If so, bypass the diagnostic work and get a new T-stat, stat. :)
I don’t have the diagnostic trees handy, but here are a few things to consider.
The O2 code is for the heater circuit, which warms up the sensor. This circuit does not measure exhaust gas. If the situation is as you hypothesize, you’d be more likely to get a specific code for running rich.
I’m really not sure what the connection is between the two, but I’d definitely start with the stat. An O2 heater is much less important to overall performance, and you might just solve them both.
Would I be crazy in thinking it may just need a new thermostat (possibly this one went bad and is stuck open) or coolant temp sensor? both cheap and usually easy fixes. The coolant temp sensor went bad in my car and the engine temp indicator stayed on cold, even though it wasn’t. Took about 1 minute to replace (it just screws in the top coolant pipe on my car).
For dealing with check engine warning lights, a scan gauge is very useful. My check engine went on a year ago on my way from Boston to DC. I was able to pull the code (a 420), and call my mechanic friend, who gave me enough info that I knew I didn’t have to do anything quickly. My recollection is that he said it was either an o2 sensor or the cat converter. It turned out to be the cat converter. I will replace it just before my next inspection, in April.
On the subject of cat converter replacements, I had replaced it maybe a year before the check engine light went on, with an aftermarket converter. That is apparently a bad idea on Hondas. Anyway, my mechanic is doing the replacement free of charge for me.
I can *STILL* remember the smell from the Cat replacement on my CR-V . . . Took something like 50 miles for it to stop smelling like burnt feline from the initial burn-off of whatever it was packed in.
Mine was OE since Honda was forced to replace it after I brought it to my mechanic.
I do all and I mean ALL of my auto repairs. I’ve got a laptop with scantool software loaded, but I’ve also got a cheepie Harbor Freight OBD scanner stuck in the glove box on a couple of the cars as well. It also turns off the CEL in case you want to verify recurrence of the code. The cost was about $30-$40 I think it was and well worth not having having to take the car somewhere to get an initial idea of a CEL.
Bear in mind these scanners will read the generic POxxxx codes, but there are also mfg-specific P1xxxx codes that can require more specialized software. However, in most cases, the generic reader will at least point you in the right direction.
Another thing to note is that the code will give you an overall idea but sometimes can be wrongly assumed. Many of the Ford 4.6 engines will give a code for “EGR flow insufficient” and the Autozone guy will happily sell you an EGR valve. Most of the time though it is the DPFE sensor and not the EGR valve. Just an example.
“What car is this? Intereting, all the cars I have owned up to and including my 09 Subaru just have the one orange check engine light.”
The color coding applies to many lights on the dash, not just the engine related lights. For example, an electronic fault in the ABS system turns on the orange “ABS” light, while a hydraulic failure or setting the parking brake turns on the red “Brake” light. The base brakes remain intact with the orange light on, but are subject to failure or damage when the red light illuminates.
Some of these guidelines are defined in Federal Regulaiton. I’ve had occasion to read the federal regs defining the flashing check engine light requirements, and it is very specific, covering details such as light color and icon design.
Jason, Stick a piece of cardboard between the grill and the radiator. This should conserve enough heat to keep the coolant temp up with even an open thermostat. Check heat output by putting hand over heater vents with temp set to high. If car runs too hot, cut the piece of cardboard down in size to allow some airflow to the radiator. Come warm weather, remove the cardboard. This solution is over 40 years old. Good luck.
sunnyvaleca,
The check engine light coming on means there is a non emergency malfunction. if the check engine light is flashing that is an emergency malfunction and you should shut the engine off.
A flashing check engine light indicates that the engine control unit has detected a missfire severe enough to damage the catalytic converter.
The days of “Get a code, get a part” are long gone. Simply throwing parts at a problem based on the code is a waste of time and $$. Diagnosis is the key here.
When I get codes for O2 Heater Malfunction, the usual cause is a defective relay that powers the relay, and in some cases I’ve had bad grounds and wiring issues to and from the relay.
My 04RAM Hemi has had me (and apparently my dealer) scratching heads for the last 20k miles. Unfailingly, after a gas fillup (no – not overfilled. stopped at click), the engine will stall for at least day at every traffic light, stop sign, or slowing to make a turn – an especially hazardous situation since both the power steering and brakes are all but gone. The truck has been to the dealer at least 8 times for this condition. Computer runs have resulted in cleaning the injectors, replacing the gas vapor tank? (TWICE), changing the CO2 sensor (TWICE), and other incomprehensible items. The last answer was to replace the gas tank itself. Why, I asked? “We think that will cure it.” Wonderful! Spend another $500 bucks and no guarantee the issue will be fixed!
Mind you I’ve built and raced vehicles of various types since I was 8 years old (now 63), my grandfather had a garage as did my dad. I grew up around cars. Yet – I can’t do a damned thing on this generation of computer controlled crap, and the dealers and their $98/hour “technicians” apparently can’t either.
Brewster, I had an 05 PT Cruiser GT that was doing all sorts of odd crap like that. Ended up being the ECM itself. The dealer had it for 2 weeks before they came to the same conclusion I did. I was getting so many codes it had to be the actual control module. That is a federally covered replacement FYI, eight year/80K miles since it emissions so your dealer should cover the fix.
Not saying your troubles are this but it sure sounds familiar. I sure miss cars that had parts like accelerator cables and what not. When it fails my the car just won’t go instead of shooting me off into a ditch at 120+ miles an hour. I find the the OBD codes so vague that they just piss me off. It gives me a handful of things to look at, some of which may or may not be what is the actual problem.
The thermostat sticking open is a common cause for codes like this. In nine years and 185,000 miles, the only non-gas-cap-related code my ’01 Grand Marquis has ever thrown was one similar to this O2 heating code (in January ’09, during a 50-mile drive on one of the coldest nights of the year).
After checking out the diagnostic tree in my Ford Service Manual, I invested $10 in a new OEM thermostat, spent less than five minutes replacing it (it’s ridiculously easy on two-valve 4.6’s), re-set the code with a scan tool, and over a year later, I’m still code-free.
As someone who’s had a thermostat stick closed (and blown head gaskets due to the sudden overheating that ensued), stuck-open thermostats that throw codes seem like a pretty good problem to have.
Crash80,
I guess I’ll have to look into that. Getting the dealer to replace the ECM without a code that “justifies” it will be a problem, though. I had to buy an extended warranty last year to cover a few other problems they busted my shoes on. Ever heard of having to remove an entire dashboard just to replace the flapper doors for a heater? $1600 bucks for that little gambit – luckily covered by the new warranty except for the $200 deductible. Years ago that would be a 20 minute job!
I know what you mean about cables and such. I long for the days when you could tune up your car with a matchbook cover to set the points and a dwell meter, putting your hand on the fender to feel engine roughness while adjusting. This is “technology”, right? Tear half an engine apart to change the spark plugs. Progress my ass.
I understand but at some point the dealer is going have to quit just throwing it’s parts and your money at it without some results. Good luck with it. I remember how frustrated I was with my Chrysler when it was acting up, almost swore off domestics altogether, though this behavior can happen with any care these days, made here or there.
I had an issue with a vehicle throwing a code. We replaced the sensor, it came back a few weeks later. Turns out, it was a bad catalytic converter. The codes really only point to an area and not a defective part.
Maybe apples and oranges, but my 88 BMW has a relay that powers the O2 heater. Just for grins I would swap in another relay that has the same pinout. In fact, I might do that to Marina’s car as a WAG to fix it’s cold stumble. My 350k mile BMW has its original cat
It seems to me the easiest an most logical thing to do is to put a oil pan heater on the transmission pan. It won’t fix the probem as some tranny fluid will still be in the cooler, tourque converter and lines but it will make warm up quicker