P0172 Fault on 2000 Honda Odyssey: Diagnosis and Common Causes

Could you provide more details about the vehicle's mileage, service history, and whether a tune-up has been performed? What is your current fuel pressure reading? Are there any known vacuum leaks?

The vehicle averages around 21 miles per gallon over 102,000 miles. I have not observed any performance changes recently. I've replaced the spark plugs once or twice and have done routine air filter replacements myself.

I haven't found any obvious vacuum leaks. However, I can't rule out a hidden leak. I don’t currently have a fuel pressure gauge—would a drop in pressure affect only one side of the engine? If so, that might explain why only Bank 1 is showing P0172.

Not necessarily—fuel system issues can be asymmetric due to differences in intake or exhaust routing. A single-side problem may still occur even without a direct connection between both sides of the engine.

I'm making an educated guess (SWAG) based on the fact that only Bank 1 is affected. This suggests issues specific to one side—such as intake gasket leaks, faulty O2 sensors, or clogged fuel injectors. Since there's no MAF sensor in this model and the EGR has been checked and appears functional, I suspect either an intake leak or a misfiring cylinder. I believe the O2 sensors are categorized as primary (in exhaust pipe) and secondary (in catalytic converter). The primary sensor shows a flat baseline with occasional spikes; the secondary remains mostly stable but occasionally dips. Both operate within 0.075–0.995 range. I plan to conduct a vacuum leak test using a manometer while monitoring fuel trims at idle, especially around intake gaskets and hoses.

SWAG stands for 'Scientific Wild Ass Guess.' Many users misunderstand Bank 1 vs. Bank 2—thinking that if only one side is affected, it must be a shared issue. But in some engine designs (especially inline engines), Bank 2 may still trigger codes even without dual-side symptoms. For P0172, the presence of a single bank code doesn’t rule out intake leaks or other localized issues. Even with a clean MAF and proper fuel pressure, possible causes include faulty O2 sensors, intake manifold gasket leaks, fouled spark plugs, EGR problems, or malfunctioning fuel injectors. Don't overcomplicate the code definition—focus on fundamental diagnostics first.

SWAG = Scientific Wild Ass Guess. I plan to replace the EGR valve gasket since it was previously leaking and the symptoms appeared before removal. However, I don’t recall a prior code that might have been related. All tests were conducted at idle with no throttle input. The manifold pressure readings from my scan tool range between 3.8"Hg and 29"Hg—within expected limits. I believe the O2 sensors are divided into primary (in exhaust) and secondary (in catalytic converter). The primary sensor shows a flat baseline with spikes; the secondary is mostly stable but occasionally dips, within the 0.075–0.995 range.

EGR systems are common issues on many Honda models—especially older V6 engines. Be sure to check fuel pressure and confirm whether a tune-up has been performed recently.

By 'tune-up,' I mean any maintenance involving spark plugs, ignition coils, or other engine-related components. No one else has worked on the engine—only me. I’ve replaced oil and air filters, PCV valve, and spark plugs. I've also replaced rotors, brake pads, and a caliper—though I suspect those were unnecessary due to repeated issues at dealerships. I’m now concerned about needing a timing belt replacement—this is a 3.5L V6 engine with high mileage.

The EGR valve isn’t the issue here—even though it was one of the first things dealers asked about. Just because it moves when voltage is applied doesn't mean it's not hanging open or leaking. Since there’s no MAF sensor and no visible vacuum leaks, could a fuel converter (such as a torque converter) be involved? That seems unlikely but worth considering.

According to the service manual, EGR valve testing is complete and passes. There is no MAF sensor on this model—only a MAP sensor. The manifold pressure reading of up to 29"Hg is within normal range. A visual and auditory inspection revealed no obvious vacuum leaks. Could you clarify which 'converter' you're referring to?

I meant the torque converter.

That seems like a stretch—though I don’t fully understand what the TCS light is trying to communicate. Could it be indicating an underlying engine issue?

The TCS light is not necessarily indicative of a traction control fault—it often activates when a check engine code exists, as a way to prompt users to inspect the vehicle. It's similar to how airbag lights come on even when no actual deployment occurs. A vacuum leak test should be performed more effectively by spraying brake cleaner around intake gaskets and hoses while monitoring fuel trims at idle. If fuel trim values fluctuate dramatically, it indicates a vacuum leak or intake contamination—especially near the manifold or intake system.