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1999 Honda Civic LX with P0170 and P0172 OBD2 Codes: Causes, Diagnosis, and Fixes

Model: 1999 Honda Civic LX Fault Code: P0170 Posted: 2016-08-13 17:05

I recently purchased a 1999 Honda Civic LX with 160,000 miles on the odometer. The car runs well and has excellent fuel efficiency, but the check engine light came on unexpectedly. Using an OBD2 scanner, I found two diagnostic trouble codes: P0170 and P0172. The vehicle previously had new oxygen sensors and spark plugs and wires installed before I bought it. However, during a test drive, we discovered one spark plug wire was disconnected — meaning the engine was running on only three cylinders. The previous owner wasn't sure how long this irregular operation lasted, which raises concerns about whether one of the O2 sensors has failed again. Additionally, when we first filled up with gas at the station, the car would not start — it would crank but fail to ignite. Once it finally started, it stalled immediately after shifting into gear. On subsequent attempts, if I kept my foot on the accelerator and gently engaged drive, it would start and run smoothly. After that, no further starting issues occurred. This behavior repeated during our next fuel refill. I'm seeking clarity on what might be causing these symptoms — especially since the P0170 and P0172 codes are consistently triggered. I’ve researched online and learned that these codes indicate a rich air-fuel mixture, but I’m not certain whether this is due to a faulty sensor, fuel system issue, or another underlying problem. Any advice on diagnosis, troubleshooting steps, or potential fixes would be greatly appreciated.

Related fault codes

P0170P0172

Comments (4)

Anonymous 2016-08-13 20:31

A cylinder without proper spark and fuel injection can lead to oil dilution and reduced catalytic converter efficiency. Use a scan tool to check O2 sensor readings, review fuel trim values, and examine freeze frame data to identify when the codes are triggered and whether engine parameters are within normal ranges. Also, try resetting the codes — if they return shortly after, it suggests an ongoing issue. As a starting aid, while in park with your foot on the brake, hold the accelerator pedal fully down for 5 seconds before attempting to start. This can help clear excess fuel from the system and improve ignition success.

Anonymous 2016-08-17 10:01

We cleared the codes successfully, but they returned within about 60 miles of driving. After changing the fuel filter and refueling again, the issue repeated. I’ve been researching and have heard that a faulty charcoal canister purge valve might be causing starting problems after refueling — though it’s a $200 part, we’d like confirmation before investing. Does this sound plausible? Is there any way to test the purge valve without replacing it first? Also, AutoZone has indicated these codes may not be directly related to fueling issues. They suggested coming in for inspection and said that if the purge valve were faulty, a separate code (like P0420 or P0441) would typically appear — which we don’t have.

Anonymous 2016-08-17 10:03

Thanks for the tip! I’ll try holding the accelerator to the floor while in park with my foot on the brake before starting. This might help resolve the stalling issue during initial starts.

Anonymous 2016-08-17 12:22

P0170 and P0172 indicate that the engine control module is detecting a rich air-fuel mixture but has reached its limits in adjusting fuel trim. These codes are commonly linked to issues such as faulty oxygen sensors, clogged intake systems, or problems with the evaporative emissions system (like a failing purge valve). Before replacing any components — especially expensive ones like a $200 purge valve — consider performing basic DIY inspections: clean the throttle body, inspect for vacuum leaks, and check fuel injector performance. If you don’t have access to a scan tool, it’s wise to get a professional diagnostic first. Affordable OBD2 scanners compatible with smartphones are available and can help identify real-time data before part replacement.