2017 – 2018 Honda CRV P0172

What causes a P0172 on a Honda CR-V?

The 5th generation Honda CRV sports a new engine option. It’s a direct-injected turbo-charged 1.5 liter, which somehow manages to push the 3,500 pound vehicle around at a reasonable rate, despite its tiny size. My wife likes camping and wanted a car for fire trails and to sleep in when the weather got bad, and the CR-V seemed like a pretty good fit since the seats fold down flat.

I’m a mechanic, and I fix cars all day. The absolute last thing I want to do with my time off is fix a car. I sometimes enjoy modifications and performance tuning, but I hate it when cars break and I have to spend my evenings or weekends fixing them. That’s why when my wife chose the CR-V, I was OK with it. Honda makes pretty reliable cars, with a few exceptions.

Oops.

I’ve always advised my customers to not buy the first year of a new generation. I should have taken my own advice. On the first oil change I was surprised how black and thin the oil was. The CR-V runs 0w20, which is thin by nature. But the oil I drained was too thin even for 0W20 and smelled strongly of fuel. I should mention that I did the oil change at 3,000 miles, far earlier than the maintenance minder recommends. I still had about 60% oil life according to the indicator.

Oil getting diluted with fuel seems to be an issue we’ll need to get used to until the engineers get direct injection dialed in. The Mazda SkyActiv direct injection engines also tend to foul oil quickly and the oil level goes up between oil changes instead of down. At this point I wasn’t too worried. I just made a mental note to change the oil more often.

Uh Oh

Fast forward about 65,000 miles. My cell phone rings and my wife’s on the other end telling me the lights are on. What lights? “All of them”, she says. When she pulled in I had to agree. There were at least six warning lights on an several warning messages flashing on the information screen. It was just like Reno without the oppressive fog of cigarette smoke and constant dinging noises.

A not-so-quick check with the Honda HDS scanner and a fair amount of research revealed that the primary issue was a single PGM-FI code: P0172. Everything else was what I call a “me too code”. You see, modern cars have a lot of networked computers that are dependent on each other. For instance, the VSA (vehicle stability assist) cooperates with the ECM (engine control module) to reduce engine output when a wheel breaks traction. So, if the engine has a problem, it may affect the VSA. When an engine code sets, the VSA sets a code too, not because the VSA has a problem, but because the ECM has found a problem.

Honda has taken this to the extreme. Is it really necessary to flash the TPMS (tire pressure monitoring system) warning light when a trouble code is stored in the ECM? I’d argue that it isn’t, and that it’s counter-productive. If all of the lights come on for every problem, what’s the point in having separate lights? There’s no way to tell from the driver’s seat which system is having the problem because all of the systems are screaming warnings at the same time.

Honda CRV P0172. What Does it mean?

P0172 is a relatively rare trouble code. It means that the engine is running too rich. What’s rich? Well, engines run best with certain ratios of air to fuel. For the best emissions, 14.7 parts air to one part fuel is ideal (14.7:1). Rich is when the ratio has more fuel than ideal. For instance, 12:1 would be a rich mixture.

The ECM uses the MAF (mass air flow sensor) to measure the amount of air going into the engine, and adds fuel by controlling injector on-time. There is an ECM “fuel map” created when the car is designed that matches injector on-time to MAF output. This works fine for the most part, but every engine is a little different, and there are small variances in the sensor calibration, so there’s a way to double check that the map is meeting its goal of 14.7:1. This is known as the feedback system.

Feedback loop

The feedback system uses air fuel ratio sensors in the exhaust to measure the air fuel mixture. If the ratio measured is 14.7:1, no changes are made to the fuel map. However, if the ratio is lean or rich, the base fuel map is modified to obtain the desired air fuel ratio. This is known as “fuel trim”. There are two varieties of fuel trim: “short term” and “long term”. Short term fuel trim is always adjusting the amount of fuel being added in real time. If a certain RPM/Load range is always running rich or lean, long term fuel trim is modified. Long term fuel trim is memorized by the ECM. Short term only responds to what’s happening at the moment.

A P0171 or P0172 sets when fuel trim needs to do too much correcting. How much is too much is set up by the engineers when they design the car. Too much is usually an adjustment of 25%-35% in either direction. If the engine is running too lean, and the ECM needs to add 40% more fuel, a P0171 will set. If the engine is running too rich and the ECM needs to reduce fuel by 40%, a P0172 will set.

I started this section by saying that P0172 is relatively rare. It’s far more common for the ECM to need to add extra fuel than it is for it to need to take away fuel. Leaks in the intake system can lead to “unmetered” air entering the engine. Since the ECM doesn’t know the air is entering, it doesn’t add fuel. Also, MAF sensors usually fail by reporting less air than what’s really flowing. Anyway, P0171 is one of the top 5 most common codes. P0172 may not even make the top 30.

There’s an engineered solution

When you search Honda’s technical information website for a trouble code, all results related to the code are displayed, which is nice. In the case of the Honda CR-V, there is a TSB (technical service bulletin) that addresses this CRV’s P0172. The service bulletin calls for re-flashing (reprogramming) the ECM, TCM (transmission control module), and HVAC (heating, ventilation, and air conditioning) control unit. Why the TCM and HVAC? I have no idea. That’s the thing about Honda — they’re very tight lipped when compared to other manufacturers like Toyota.

Whenever there’s an applicable TSB, it should always be your first step. Fixing a design flaw is impossible in the field. Even if the issue isn’t a design flaw, and the TSB is only guidance in repairing a common problem, it makes sense to try the TSB before doing anything else. So, I hooked up the HDS scanner and proceeded to flash the ECM and TCM. However, there was no option to re-flash the air conditioning control unit. A little more research revealed that some CR-V EX and EX-Ls had an A/C unit that cannot be flashed, an that the unit should be replaced instead. $100 later I had installed a new A/C control unit. The price was actually very reasonable. I’ve paid nearly $2000 for some ECUs.

The final step of the TSB is to replace the engine oil. Once again, the TSB didn’t specify why, but it almost certainly was to eliminate fuel dilution. You see, some vapor blows past the piston rings into the crankcase. It’s called “blow-by” and all engines have at least some. Some of the vapor is unburned fuel. To prevent build-up of explosive vapors in the crankcase, a ventilation system is used. Vapors are sucked into the intake manifold and then on into the combustion chamber to be burned. If the oil is heavily diluted with fuel, the vapor can make the engine run rich, since so much extra gas is being added through the PCV (positive crankcase ventilation) system.

The TSB didn’t fix the problem

Following the TSB fixed the issue for a while, which was probably due to changing the oil. The oil on this car fouls with fuel very quickly, and the fuel in the oil is sucked through the PCV system and into the intake, adding “extra” fuel and causing the P0172. Unfortunately reprogramming the ECM, TCU, and replacing the AC control unit did nothing to slow the fuel fouling of the oil.

Honda issued a warranty extension for the engine damage that can be caused by the fuel-thinned oil, but my wife’s engine was still fine, probably because I change my oil way more frequently than Honda recommends. 

THE FIX

What was the fix? Well for me it was a new Prius Prime. I threw a camper shell on my truck for my wife to take camping and she traded in the CRV for the Prius. Did I mention that I hate fixing my own cars in my spare time? The 2017-2018 CRV with the 1.5 turbo is a poorly engineered pile of crap. If a customer had brought the car in, I’d be forced to figure it out, but since it was my own car I was under no such pressure. 

What do I suspect it was? If you search “TGDI” (turbo gasoline direct injection) and “fuel dilution”, you’ll find that gasoline condensing and sliding past the piston rings causing fuel dilution is one of the engineering challenges with this type of engine, especially on high output engines.

Direct injection engines foul oil, and this one was worse than most, which was evident from the very first oil change. However, P0172 wasn’t a problem for the first 65,000 miles on this CRV, so what changed? I think it was the fuel injector spray pattern. I think over time the nozzles clogged and instead of delivering a fine mist, they sprayed streams and droplets causing liquid fuel to pool and dribble past the piston rings. 

It’s likely I’ll be forced to find out fairly soon though. My wife drives around 20,000 miles a year, so she hit 65K a lot sooner than most CRV owners. I can’t imagine that her CRV will be the only one with this issue, so I’m expecting a flood of Honda CRVs with P0172 any day now. Meantime, if you own 2017-2018 CRV, or any car with a gasoline direct injection engine, do yourself a favor and change your oil frequently.

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