BMW E46 318i (1998) P0300-P0303 & P0130/P0170 Fault Codes: Root Cause Diagnosis and Fixes

I recently encountered similar codes on my BMW E46. The car runs rough at idle but smooths out when accelerating. I checked the idle control valve (ICV) and found it was dirty — cleaned it thoroughly. Also, I discovered a large hole in the intake manifold vacuum hose near the throttle body. Replacing that fixed most of the misfire issues. For reference: [bavauto.com/shop.asp](http://www.bavauto.com/shop.asp), [bimmerforums.com/forum/sh...p?t=900798](http://forums.bimmerforums.com/forum/sh ... p?t=900798). Hope this helps you resolve it!

Hi David, thanks for the tip! I'll check my idle control valve next. The car does feel rough at idle — not funny, just noticeably inconsistent. Once I accelerate, it runs fine and stays stable until I return to idle.

If your engine idles poorly but runs well under load, this strongly suggests an air leak in the intake system. Check all vacuum hoses and connections — even small leaks can cause misfires and fuel trim issues.

Yes, I'm considering that too. I'll inspect all vacuum lines first before moving on to cleaning or replacing the MAF sensor. This weekend (weather permitting), I’ll do a full visual inspection of hoses and connections. Will update if anything changes.

Hi everyone — here's an update: I found that the breather hose from the Carter system was completely torn at both ends. Replaced it along with several other vacuum hoses in poor condition. After repairs, the car runs smoothly again. I reset all codes but shortly after a few miles, P0170 returned. Freeze frame data shows high long-term fuel trims (Bank 1: +19.5%, Bank 3: +30.5%). The rear O2 sensor (after the catalytic converter) is now fluctuating irregularly — unlike the front O2, which has stable readings. I suspect this sensor may be faulty or affected by a previous leak. I’m still investigating whether an air leak could be contributing to the issue. Any suggestions on how to verify rear O2 sensor behavior under normal conditions would be helpful. Thanks! Vincy

My '97 BMW 528i also shows P0170 — Bank 1 rich mixture (cylinders 1, 2, 3). After a head gasket repair and cylinder 3 crack fix, I drove over 1,000 miles without issues until it returned during a snowy morning when temperatures dropped below freezing. The code came back after 100 miles. I was told that water in cylinder 3 caused the O2 sensor to freeze — now it just needs replacement. I’ve replaced both front and rear O2 sensors (same model), but will monitor performance. If this helps, I’ll share results. Any input is appreciated.

For clarity: the front O2 sensor should fluctuate roughly once per second between ~0.2V and 0.8V. The rear O2 sensor (after catalytic converter) should read low — typically near 0.1–0.3V when operating normally. While front sensors affect engine performance, rear sensors primarily monitor exhaust conditions and don’t directly impact drivability. If the rear sensor is faulty or misbehaving, it may cause incorrect fuel trim readings without affecting running quality. This helps explain why P0170 appears — a misreading from the rear O2 could lead to over-fueling.

I have a similar issue on my 2000 E46 M43 engine. I've replaced nearly all breather hoses and both O2 sensors, cleaned the MAF sensor and engine temperature sensor. The car still runs erratically and smells slightly rich — no CO detected in emissions tests, but lambda readings are high. After certain RPMs, it seems to stabilize briefly, then worsens after a 50-mile run. Recently, I lost spark on cylinder 3 — previously had spark on all cylinders. This could indicate a wiring issue or faulty ignition coil. I suspect the problem may be electrical rather than mechanical. Anyone else have similar experiences with missing spark and P0303 misfire? Any suggestions would be helpful.