Multiple codes

[#1 demolition]

i have an 03 baja that has p0122 p0512 p1518 p0852 p0851 p0138 p0137 p0502 p0503 they all came on at the same time has anyone else had this happen

 

[bmart]

If only someone made a search engine you could use...

https://www.wrxtuners.com/threads/suddenly-getting-codes-p0137-p0851-p1518-p1152-p1153-p0502-p0503.73905/

These specific diagnostic trouble codes frequently appear together in a cluster on Subaru vehicles (commonly in 2002–2007 WRX, Legacy, and Outback models). This combination strongly points to a main wiring harness ground fault, a shorted sensor, or damage from aftermarket modifications rather than simultaneous failures. [1, 2, 3]
The combination of your specific codes reveals several related issues:

• P0122: Throttle/Pedal Position Sensor Circuit Low Input.
• P0137 & P0138: Downstream (Rear) Oxygen Sensor Circuit Low Voltage / High Voltage.
• P0502 & P0503: Vehicle Speed Sensor (VSS) Circuit Low Input / Intermittent/Erratic.
• P0512 & P1518: Starter Request Circuit / Starter Switch Circuit.
• P0851 & P0852: Neutral Switch / Park-Neutral Position Input Circuit Low / High.
• P0502/P0503: Vehicle Speed Sensor. [1, 2, 3, 4, 5]

Common Root Causes

• Bad Grounds: Many of these circuits share a common sensor ground. If the ground wire has corrosion, a loose connection, or was compromised (especially after an aftermarket alarm, remote start, or audio system install), it throws all of these sensors out of range at the same time. [1]
• Shorted O2 Sensor: A shorted rear oxygen sensor can pull down the 5-volt reference voltage shared by other sensors (like the Throttle Position Sensor). [1, 2]
• Starter/Inhibitor Switch: A faulty or disconnected starter relay wire or Park/Neutral safety switch will trigger the starter circuit and range codes. [1]

Diagnostic Steps

1. Check for Aftermarket Wiring: If your vehicle has an aftermarket remote starter, stereo, or alarm system, verify that none of the connections are interfering with the starter switch or neutral safety wires. [1]
2. Clean and Inspect Grounds: Check the main engine grounds and the grounds under the dashboard. Corroded or loose grounds are the #1 cause of this specific code cluster. [1]
3. Check the Battery and Connectors: A failing battery or a dirty, loose connection at the Engine Control Unit (ECU) can result in low voltage spikes that trigger these multiple circuit faults. [1, 2]
4. Inspect the O2 Sensor: Unplug the downstream O2 sensor. If the other codes (like P0122) disappear, the O2 sensor has internally shorted and needs replacement. [1, 2, 3, 4]

For tips on how to check your vehicle's wiring harness and ECU connectors for shorts and ground issues:

2:08


How I Diagnosed This P0122 Check Engine Light ...


Online Mechanic Tips
YouTube · May 13, 2026