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1998 Mitsubishi 3000GT P0173 and P0170 Fuel Trim Issues: Diagnosis and Fixes

Model: 1998 Mitsubishi 3000GT Fault Code: P0170 Posted: 2011-08-19 18:24

Hi everyone! I'm new to this forum and have been searching for help with a persistent issue on my 1998 Mitsubishi 3000GT. The problem has been ongoing for over a year, and I'm now looking for reliable insights from experienced users. Vehicle Details: - Model: 1998 Mitsubishi 3000GT - Engine: 3.0L V6 SOHC 6G72 - Mileage: ~185,000 miles - Transmission: Manual I've been consistently seeing OBD-II fault codes P0173 and P0170 — both indicating fuel trim malfunction in Bank 1 and Bank 2. The check engine light typically activates during light throttle cruise (at speeds like 40, 50, or 55 mph), and I've noticed it's coming on more frequently now — often appearing after the second drive cycle following a code clear. Current Fuel Trim Readings: - Long-Term Fuel Trim (LTFT): +12.5% (Bank 1) - Short-Term Fuel Trim (STFT): 9–16% (both banks) I've already performed the following troubleshooting steps: - Replaced both oxygen sensors (O2) in Banks 1 and 2 — previously triggered O2-related codes, so I replaced them. - Conducted a smoke test on the intake system, identifying minor vacuum leaks. All vacuum lines have been repaired and replaced. - Verified coolant temperature sensor function — readings appear normal. - Confirmed O2 voltage fluctuations between 0.8V and 0.1V (within expected range). - Fuel pressure is stable: 36 psi with fuel pressure regulator (FPR) under vacuum, rising to 48 psi when disconnected (within spec of 48–50 psi). - FPR responds correctly — pressure spikes when the vacuum line is connected or disconnected. - Tested three different airflow sensors — all show idle readings between 3.3–3.8 g/s, which is below the factory specification of 4.7–12 g/s. I’ve confirmed there are no exhaust leaks before the O2 sensors, so I don’t believe unburned air is being drawn into the exhaust stream. The car runs smoothly and performs well under normal conditions — only the check engine light and fuel trim codes remain. The vehicle has been lightly modified: aftermarket air filter, deleted pre-catalytic converters (pre-cat), but with an intact main catalytic converter. I’m currently using spark plugs that are colder than stock heat range — I plan to switch to proper heat range soon. Could this be affecting fuel trim behavior? I’ve heard suggestions from a local shop with 30+ years of experience that the issue might stem from faulty or degraded fuel injectors, especially given the high mileage. However, since the car runs well otherwise, I’m skeptical. Could anyone recommend additional diagnostics or tests to help identify the root cause? Any insights on potential sensor types (e.g., wideband vs. narrowband O2 sensors), intake airflow calibration, or other system-level issues would be greatly appreciated! Thanks in advance for your time and advice — I really appreciate it!

Related fault codes
P0170P0173
Comments (3)
Anonymous 2011-08-19 22:35

It's important to note that removing pre-catalytic converters (pre-cats) can disrupt the air-fuel ratio feedback loop, potentially causing fuel trim errors like P0173 and P0170. I recommend restoring the vehicle to its original stock configuration with all pre-cats reinstalled — this may resolve the issue. Also, what brand of O2 sensors did you install? Some aftermarket brands may not provide accurate readings or are incompatible with the 6G72 engine's control system.

Anonymous 2011-08-20 08:36

I didn’t specifically mention it to the shop — both O2 sensors I installed are DENSO, which was the original equipment manufacturer (OEM) for Mitsubishi. While DENSO is a known brand, it’s worth noting that simply replacing with the same brand doesn't guarantee compatibility or accuracy. Some aftermarket O2 sensors may be narrowband only, while the system expects wideband sensors for precise air-fuel ratio control — especially in older V6 engines like this one.

Anonymous 2011-08-20 11:01

A common oversight when replacing sensors is choosing a standard narrowband O2 sensor instead of a wideband one. Even if the brand matches, using an incorrect type can lead to inaccurate feedback signals and persistent fuel trim errors like P0173/P0170. For optimal performance and accurate diagnostics on a 1998 3000GT with a 6G72 engine, wideband O2 sensors are strongly recommended.